This is an incomplete list of some useful security tools.
Forensics
FTK Imager NTFS imager that lets you browse all data in an NTFS partition. Useful for discovering NTFS-specific data such as security IDs of computers a partition has been connected to.
Volatility Memory forensics tool
Binwalk Looks for headers inside of files to find data structures or files inside of files.
Foremost Searches for files inside of files based on their headers, but you must know what you are looking for.
ExifTool
View, edit, or delete exif data in files. Better alternative to the normal exif tool.
zsteg Steganography tool for discovering files inside PNG and BMP images.
FLOSS
FireEye Labs Obfuscated String Solver. Alternative to the normal strings command and automatically tries to deobfuscate strings in a binary or file.
Web Exploitation
SQLMap Automatic SQL injection
Reverse Engineering
OllyDBG Binary code analyzer and disassembler
x64dbg x64/x32 debugger for Windows
Radare2 Disassembler and debugger