Syrinscape - Sound effects and ambient music mixer
Tabletop Audio - Original, 10 minute ambiences and music for your games and stories.
Character Creation
CharacterTool - An open-source tool for players to manage their characters during game play. It is fully scripted so that the tediousness of tracking large numbers of stats can be automated.
Map Making
MapTool - An open-source tool that includes a wide range of resources for creating detailed maps. Its primary role is to work as a Virtual Table Top that replaces pen, paper, battle mats, and dice.
Fantasy-Map-Generator - An open-source tool which procedurally generates fantasy maps. You may use auto-generated maps as they are, edit them or even create a map from scratch.
Remote play
Tools to host a session remotely
Roll20 - Web platform to host games online. Includes dynamic char sheets and a Marketplace with integrated content for several systems.
Fantasy Grounds - Desktop virtual tabletop app. Includes, among others, DnD, Call of Cthulhu and Pathfinder content.
Astral TableTop - Web platform to host games online. Includes integration with Dungeon Masters Guild and DriveThru RPG.
Owlbear Rodeo - Web platform to host games online. Very lightweight and easy to use. Does not require players to create a login.
The Dholes House - Web platform to host Call of Cthulhu games online. Users can also create character sheets and share materials with other players.
Resources
Icons
game-icons.net - 3400+ Creative Commons icons in PNG and SVG.
Miniatures
Paper
Printable Heroes - PrintableHeroes creates print-ready miniatures (or minis) for Dungeons and Dragons or other table top role-playing games and then releases them for free each month.
Paper Forge - Fantasy paper minis, item cards, spell cards and other props for use in D&D, Pathfinder and other Tabletop RPGs
3D
Hero Forge - Design your miniatures choosing from a library of body parts and props and download or order 3D prints. You can also take screenshots to use as hero portraits/tokens.
Miscellaneous
RPG Cards - Fantasy card generator. Generate items, spells, monsters, characters, or NPCs.
D&D Beyond - Official digital toolset for D&D, nice help to mount your character's sheet.
Improved Initiative - Combat planner and tracker with player and DM views, with conditions tracking and detailed creature info (great for use with tablets)
Kobold Fight Club - Encounter planner and runner. Focused on building balanced encounters.
GURPS 4e
Tools
Character Sheet
GURPS Character Creation - A stand-alone, interactive, character sheet editor that allows you to build characters for the GURPS 4th Edition roleplaying game system. It is constantly kept updated for ruleset compatibility.