Google Chrome or Firefox Dev Edition
Inspect element
Cookies - Chrome Edit this cookie or other browser equivalent
Browser Developer tools
Burpsuite - Web Proxy
SQLMap - Automated exploitation of SQL Injection Vulnerablities
NMap - Port Scanning
RequestBin - Receive requests
Shodan - IOT Search Engine
strings - Print the strings of printable characters in files
ltrace - A library call tracer
ptrace - Process trace
pwntools - CTF framework and exploit development library
gdb-peda - Python Exploit Development Assistance for GDB
Binary Ninja or IDApro - Reversing Platforms
Hopper (64bit) - The macOS and Linux Disassembler
Java Decompilers - Java decompiler, many of these exist
Uncompyle2 - Python 2.7 Decompiler
x86 Instruction Set Reference
radare2 - Unix-like reverse engineering framework and commandline tools
Angr - A binary analysis framework
Ropper - Build rop chains
quipquip.com - fast and automated cryptogram solver
dcode.fr - Decoding tools
lantern - Cryptanalysis library for breaking classical ciphers
Factordb - Prime Factorisation
Pycrypto - Collection of hashing and encryption algorithms
CyberChef - Recipes for data manipulation
Crackstation - Pre-computed lookup tables to crack password hashes
hashcat - Hash Cracker
John The Ripper - Password Cracker
pdf2john.py - PDF password extraction
fcrackzip - Cracking encrypted ZIP archives
Binwalk - Firmware Analysis Tool
Foremost - Recover files using their headers
Stegsolve - Image steganography helper
Audacity - Sound editing
GIMP - GNU Image Manipulation Program
file - Determine file type
mount - Mount a filesystem
Sleuthkit - Open source digital forensics
Wireshark - Network protocol analyzer
Rekall - Memory Analysis Framework
Your favourite scripting language (Python, Perl, Ruby, Bash, etc)
Pwntools - CTF framework and exploit development library
Google search
OSINT Framework - Information gathering from free tools
WIFI or Bluetooth SSID
Google Reverse Image Search
Google time-range feature
Wayback Machine - Internet Archive
BuiltWith - Find out what websites are built with