Tools & Resources

Info 1: http://www.sortingh.at/
This interactive guide is meant for people with no budget and no experience whatsoever in making games or programming. The guide also works for people who are working alone, though it can easily be used by teams as well.

Info 2: http://www.pixelprospector.com/game-development/
Indie games... and essential resources for game developers

Info 3: https://developer.mozilla.org/en-US/docs/Games
MDN game development center.

Info 4: http://gameprogrammingpatterns.com/contents.html Book with a collection of patterns found in games that make code cleaner, easier to understand, and faster.

Art

Gimp https://www.gimp.org/
GIMP is a free and open-source cross-platform raster graphics editor
A good alternative to photoshop to edit images/photos etc. Can paint with it too.

Krita https://krita.org/
Krita is a professional free and open-source painting program. Loads of brushes etc.

Paint.NET https://www.getpaint.net/
Another free image and photo editing software for windows.

3D

Blender https://www.blender.org/
A free & open-source 3D modeling, sculpting, and animation software

Sculptris http://pixologic.com/sculptris/
A free 3D sculpting tool, kind of the free version of zbrush.

Crocotile3D https://prominent.itch.io/crocotile3d
A tool for creating 3d scenes with tiles. Think of it as creating 2d tilemaps with three dimensions.
Free to use and save & export under 100 tiles, have to purchase it for using more than 100 tiles.

Sprytile (Blender Addon) https://chemikhazi.itch.io/sprytile
Inspired by Croctile, this open-source free Blender addon gives blender tile editing tools.

Level Buddy (Blender Addon) https://matt-lucas.itch.io/level-buddy
Level Buddy is a CSG level modeling workflow add on for Blender 3D. It takes inspiration from old school editors like UnrealEd and DoomED. It is not BSP, you are work with poly meshes at all times.

Pixexix http://pixexix.sophiehoulden.com/
It lets you draw onto your 3D models using any 2D image editor. Purchase of Pro version needed if you want bigger resolution bigger than 1024^2.

MakeHuman http://www.makehumancommunity.org/
An open source tool to create a base for 3D humanoid characters.

VRoid Studio https://vroid.com/en/studio/
A virtual youtuber character creating tool, might be neat as a base for 3D anime like characters.

Animation

OpenToonz https://opentoonz.github.io/e/index.html
Free and open-source animation production software which supports both vector & raster animating, tweening, skeletal animation, cut-out rigging, and much more.
Quite complex but very powerful, a good free alternative to Adobe Animate.

Gimp with plug-ins like Narly Sprite https://github.com/narlynephi/narly_sprite
Gimp with this plugin allows multiple layers per frame, duplicate frames, exporting to sprite sheet or flattened layers with frame number for gimp gif exporting, previous frame transparency (primitive onion skinning), and some more things.

Krita https://krita.org/
Has frame-by-frame raster animation, with very customizable onion skinning.

Paint.Net with plugin https://pixelbyte.itch.io/paint-net-sprite-plugin
Can help out with sprite sheet editing and save as gif too.

Rive https://rive.app/
A free cloud based animation webapp featuring Vector and Raster support, skeletal, mesh deformations, IK etc. An account allows for unlimited public files, but private files needs a paid plan.
It also includes runtime implementaitons at https://rive.app/runtimes

DragonBones https://docs.egret.com/dragonbones/en/index.html
A free 2D bone animation tool/editor, features skeletal and skinning/mesh + IK and timelines etc. It's a great free alternative to Spine. It has integrations for: SFML(C++), ThreeJS + Pixi + Phaser (JS & TypeScript), unity, and GoDot plugin and more. Also has tools to convert to Spine json, Live2d json with these tools: https://github.com/DragonBones/Tools
Find more about their runtime integrations at https://github.com/DragonBones

Live2D Cubism https://www.live2d.com/en/download
Is an animation tool that allows illusion of 3D from 2D characters, and provides ability to adjust their emotions and movements allowing for characters with detailed expressions, great for visual novels.
It has free or pro version, free version is a limited version of the commercial one with limitations on resources etc.
They also provide the SDK to integrate with unity, native, or web.

AnimeEffects http://animeeffects.org/en/
It is an open source 2D keyframe animation tool based on deformation of polygon meshes.
Exporting animations to an animated GIF, or sequence PNG/JPEG. Or Video formats like WEBM and MP4 etc. using FFmpeg (need to download it seperately).
Their github: https://github.com/hidefuku/AnimeEffects

Anima2D (Unity plugin) https://assetstore.unity.com/packages/essentials/unity-anima2d-79840
Has 2D Bones, Sprite to mesh conversion, SpriteMesh Editor, Automatic weights, Weight Editor, Inverse Kinematics etc.

COA Tools (Blender Addon) (CutOut Animation Tools) https://github.com/ndee85/coa_tools
2D cutout animation workflow similar to programs like spine or spriter.

Pixel Art/Animation software

Graphics Gale https://graphicsgale.com/us/
Good tool for pixel art and spriting, has a free version.

Piskel http://www.piskelapp.com/
Online animated sprite and pixel art editing/creating, and you can freely download an offline version of the app too.

Paint of Persia https://dunin.itch.io/ptop
Is a free rotoscoping pixel-art tool where you can trace/draw on top of any window, wallpaper or video etc.

Aseprite https://www.aseprite.org/
Similarly also a good tool for pixel art and spriting, but you'll have to purchase it, unless you can compile the source yourself.

Pyxeledit http://pyxeledit.com/
A paid pixel art drawing application especially designed for working with tiles. It also supports making animations, and exporting them as sprite sheets or animated GIFs. It has an outdated free version.

Image/Animation Tools

Shoebox https://renderhjs.net/shoebox/
A free Adobe Air based app, sort of a swiss knife for ui or sprite sheets, it has things like sprite sheet packer & extracter, slicer, frame sheet, bitmap font, tile extractor from a screenshot of a game, and texture ripper etc.

Tiled Map Editor https://thorbjorn.itch.io/tiled and http://www.mapeditor.org/
Tiled is a free software level editor. It supports editing tile maps in various projections (orthogonal, isometric, hexagonal) and also supports building levels with freely positioned, rotated or scaled images or annotating them with objects of various shapes. Also for its Unity import support https://seanba.itch.io/tiled2unity

Explosion Generator http://www.explosiongenerator.com/
Generate very customizable 2D sprite explosions. It is a web app.

Sprite Sheet Previewer https://github.com/kobitoko/Sprite-Sheet-Previewer
A simple .Net4.6 free tool to preview sprite sheets.

SpriteMapper https://github.com/huandu/spritemapper
SpriteMapper is a Java program which creates sprite maps (or sprite sheets) from a set of input images.

FontSprite https://adamstrange.itch.io/fontsprite
Complete .png fontsprite editor. A Fontsprite is a 16x16 character grid, usually 256x256 pixels in size.
This is how the older 6bit and 16 bit computer referenced their fonts. Using this editor you can directly edit this sort of font with complete control over color and transparency.

Peasy https://yadu.itch.io/peasy
is a simple and free tool that generates polygonal hulls from transparent png images, and exports it as a JSON.
These polygonal hulls are used for more accurate collision checking in e.g. Box2D.

Pngyu https://nukesaq88.github.io/Pngyu/
Good solution to reduce PNG image sizes via lossy compression.
Pngyu is simple PNG image file compression tool which uses pngquant as a compression engine.

ScreenToGif https://www.screentogif.com/
A free and open-source software for when you need a small gif or video to show off your new game features for twitter etc.

Playscii https://jp.itch.io/playscii
Playscii is an open source ASCII art and animation program.

Tilemancer https://led.itch.io/tilemancer
Tilemancer is a quick procedural tile creator designed for pixel-art games. Perfect for game jams, prototypes, texture mods, and more.

Dither Machine https://lunarlabs.itch.io/dither-machine
Dither Machine is a tool to help you generate dithering automatically, instead of doing it by hand, which can be a tedious task.

HTML Animation and Utilities http://www.gifloopcoder.com/utils.html
You can adjust settings on each utility and it provides JS output for you to copy into your code.
It has Bezier Utility, Path Utility, Shadow Utility, Sprite Sheet Utility.
They offer also a library/app for looping gif animations on HTML/JS https://github.com/bit101/gifloopcoder

Coolors https://coolors.co/ Color schemes generator. Generates well balanced color palettes (up to 5 different colors per palette) for your game project.

Sound

Audacity https://www.audacityteam.org/
Audacity is free, open-source, cross-platform audio software for multi-track recording and editing.

bfxr https://www.bfxr.net/
A free sound effect generator, and can export the results in wav format, alternatively you can download a Standalone version, since the online version uses Adobe Flash.

rFXGen https://raylibtech.itch.io/rfxgen A simple and easy-to-use tool to generate sounds for your games, based on the amazing sfxr.
Their github is at https://github.com/raysan5/rfxgen

Chiptone http://sfbgames.com/chiptone
Free online tool (Adobe Flash) for generating sound effects. Can export the result in wav format.

Music

LMMS https://lmms.io/
A free, cross-platform tool for creating melodies and beats, synthesizing and mixing sounds, arranging samples and much more.

MidiEditor http://midieditor.sourceforge.net/
A free software to make, edit, and play Midi data.

Anvil Studio http://anvilstudio.com/
Another free software to create, edit, and play Midi data.

Trackers:

sunvox http://www.warmplace.ru/soft/sunvox/
A small, fast and powerful modular synthesizer with pattern-based sequencer (tracker), free for desktop, paid for iOS or Android mobile device versions.

Bosca Ceoil http://boscaceoil.net/
A free and easy to use tool for creating music. Online on web or download the offline app.

PixiTracker http://www.warmplace.ru/soft/pixitracker/
A simple and fun tool to quickly create musical sketches, chiptunes and sound experiments.

OpenMPT https://openmpt.org/
Open ModPlug Tracker is a free tracker for windows.

Milkytracker http://milkytracker.titandemo.org/ https://github.com/milkytracker/milkytracker
A free multi-platform music application for creating .MOD and .XM module files.

Online Audio Tools

Soundation Studio https://soundation.com/
A free powerful online music studio. Has paid plans with more features.

Audiotool https://www.audiotool.com/
A free online music production studio.

Online Sequencer https://onlinesequencer.net/
Free online music sequencer.

Autochords https://autochords.com/
Generate some chord progressions for your music.

Music Generators

A lot taken from https://www.yoyogames.com/blog/119/procedural-music-a-viable-alternative

CGmusic http://codeminion.com/blogs/maciek/2008/05/cgmusic-computers-create-music/
Software with algorithms to create midi music for you.

Soundhelix http://www.soundhelix.com/download
Free open-source tool for generating more modern music, but has a very steep learning curve.

Melody Brains http://www.melodybrains.com/
Produces melodies with backing chords. Made by Marko http://fatdzo.github.io/

MusiGenisis http://www.musigenesis.com/
Trial is fully functional but cannot export. Purchases are not available either anymore, you could try contacting them and hopefully get responses, but there possibly other solutions to capture the audio.

Fake Music Generator http://www.fakemusicgenerator.com/
Creates an entire album using CGmusic's software.

Wolfram's Tones http://tones.wolfram.com/generate/

Video Editing

Lightworks https://www.lwks.com/
a professional-quality video editor that is free for home use.

Natron https://natrongithub.github.io/
a free open-source compositing software. Node-graph based. Similar in functionalities to Adobe After Effects and Nuke by The Foundry.

Shotcut https://www.shotcut.org/
a free, open-source, cross-platform video editor.

Openshot http://openshot.org/
a free and open-source video editor.

Kdenlive https://kdenlive.org/
a free and open-source video editor.

Game Engines, Frameworks, Libs.

Unity (C#) https://unity.com/
Unreal (Blueprint(Visual Scripting), C++) https://www.unrealengine.com/
Defold (Lua, C++) https://defold.com/
GODOT (C++, C#, GDScript, Visual Scripting, community-provided support for Python and more) https://godotengine.org/
Cocos Creator, Cocos2dx (Creator: JS, TypeScript. Cocos2dx: C++, Lua) https://www.cocos.com/en/ https://github.com/cocos2d/cocos2d-x
Ren'Py a visual novel engine (Python) https://www.renpy.org/
GDevelop (Visual Scripting, extendable with JavaScript) https://gdevelop-app.com/
Game Maker Studio (Visual Scripting, GML) https://www.yoyogames.com/gamemaker
Construct 2 (Visual Scripting) https://www.scirra.com/
FELGO (formerly known as V-Play) crossplatform games/apps (C++, QML, JavaScript) https://felgo.com/
WOLF RPG Editor (Free WYSIWYG Rpg maker software) https://www.moddb.com/engines/wolf-rpg-editor
Eclipse OnlineRPG engine https://www.eclipseorigins.com/downloads
Danmakufu (Bullet Hell Engine for running user-created danmaku scripts) https://dmf.shrinemaiden.com/
Touhou Web Engine Engine for web-based bullet dodging game (JavaScript) https://github.com/kvfrans/touhou
Mugen (2D fighting game engine) https://mugenarchive.com/forums/downloads.php?do=file&id=1421
OpenBOR (2D side scrolling/Beat'em up engine) https://github.com/DCurrent/openbor
processing good for small prototypes (Processing (Java-like)) https://processing.org/
Processing in JS: https://p5js.org/ and Python: https://py.processing.org/

LÖVE (Lua) http://love2d.org/
Phaser3 or Phaser CE 2D game framework for making HTML5 games on Canvas and WebGL (JavaScript) http://phaser.io/ https://github.com/photonstorm/phaser-ce
libGDX (Java) https://libgdx.badlogicgames.com/
BabylonJS Create 3D WebGL games for the browser (JavaScript) https://www.babylonjs.com/
PaperJS A vector graphics scripting framework for HTML5 Canvas (JavaScript) http://paperjs.org/
ZIM A framework for the HTML5 Canvas, powered by CreateJS (JavaScript) https://zimjs.com/about.html

pygame (Python) https://www.pygame.org/news
SFML (C++, Java, Python and more) https://www.sfml-dev.org/
SDL2 (C, C++) https://www.libsdl.org/download-2.0.php
Box2D a 2D physics engine for games (C++, ports for: JavaScript, Java, C# ) https://github.com/erincatto/Box2D
Chipmunk2D 2D rigid body physics library written in C (C++, Python, Ruby, Java, Ports: JS) http://chipmunk-physics.net/
Liquidfun = Box2d + fluid simulation (C++, JavaScript, Java) http://google.github.io/liquidfun/
RMLUi a C++ user interface package based on HTML and CSS standards (C++, XHTML1.0, CSS2.0, borrowing features from HTML5 and CSS3) https://github.com/mikke89/RmlUi

List of HTML5 game engines https://html5gameengine.com/

itch.io list of game engines (and more) https://itch.io/blog/6037/get-ready-for-ludum-dare-game-development-tools#game-engines

Programming

HumbleNet a cross-platform networking library that works in the browser (C/C++ C#) https://github.com/HumbleNet/HumbleNet
enet ENet reliable UDP networking library (C) http://enet.bespin.org/
GameJolt Api https://gamejolt.com/help/dev-game-api
Communicates with the Game Jolt servers. Implementing the Game API allows you to add features to your game such as: Online score tables, Trophies (achievements), Cloud data storage, Game session logging, User verification and auto-login.

MSYS2 http://www.msys2.org/
Minimal SYStem 2 is a software distro and building platform for Windows. Allows you to use gcc, g++ etc in windows.

Free C/C++ Profiling/Testing:

CodeXL https://github.com/GPUOpen-Archive/CodeXL
Very Sleepy (Windows only) http://www.codersnotes.com/sleepy/
cppcheck (a static analysis tool for C/C++) https://github.com/danmar/cppcheck/
Googletests (a C++ test framework) https://github.com/google/googletest
clumsy (lag/network condition emulator) https://github.com/IntouchHealth/clumsy

Text Editors & IDEs:

IntelliJ IDEA (Java, Kotlin, Scala, Groovy and more JVM languages) https://www.jetbrains.com/idea/
Visual Studio Code https://code.visualstudio.com/
Atom (C++, C#, JavaScript, Python, and more) https://atom.io/
Notepad++ (a lot) https://notepad-plus-plus.org/
Brackets (Web Development focused) http://brackets.io/
Geany (C++, C#, JavaScript, Python, and more) https://www.geany.org/Main/HomePage
Visual Studio Community edition (C++, C#, VB, and more) https://visualstudio.microsoft.com/vs/community/
Netbeans (Java, PHP, C/C++, HTML5) https://netbeans.org/
Android Studio (Android App Development focused) https://developer.android.com/studio/index.html
Code::Blocks (C/C++) http://www.codeblocks.org/
Eclipse (Java, C/C++, and more) https://eclipse.org/
PyCharm (Python) https://www.jetbrains.com/pycharm/

Level Design

Cavegen https://elementalcode.itch.io/cavegen A Cave Generator based on Cellular Automata.

Quest Design

RapidQuest https://www.rapidquest.ai Visual development environment for designing branching game quests & narratives using AI.

Other tools

YarnSpinner Editor https://github.com/YarnSpinnerTool/YarnEditor
A dialogue editor tool used to write interactive dialogue, it has a C# and JS library for runtimes, also supports Unity.

cmder http://cmder.net/
A great customizable console emulator for Windows, it looks nice and can have tabbed terminals etc.

More Random tools on itch.io https://itch.io/tools

File Converter https://github.com/Tichau/FileConverter

QWinFF https://qwinff.github.io/
An open-source GUI for ffmpeg http://ffmpeg.org/ which is an open-source command-line media converter.

Slack https://slack.com/
A team communication app.

Trello https://trello.com/
An online board with lists and cards to keep track of tasks and projects.

OBS Studio https://obsproject.com/
Open Broadcaster Software studio is a free and open-source software for screen recording and streaming.

OpenCV (C++, C, Python, Java) https://opencv.org/
open-source Computer Vision Library is an open-source computer vision and machine learning software library.

instant storm 2.0 https://www.instantstorm.com/
Is a freeware flash screensaver creator that turns Adobe Flash files (SWF) into screensavers.