Tools are categorized by their main use, anything else goes to the bottom.

Reverse Code Engineering (RCE)

Debuggers

Cross-platform/Linux

• dbx
• GDB (GNU debugger, open source)
• VDG

Windows

• BugChecker (open source)
• ImmunityDebugger
• OllyDbg
• pydbg (open source)
• Syser Kernel Debugger (practically dead)
• Visual DuxDebugger
• WinAppDbg debugger (open source)
• WinDbg

Disassemblers/Decompilers

• biew aka beye (open source)
• boomerang decompiler (open source)
• HIEW
• Hopper - also see hopper
• IDA Pro, also the Hex-Rays Decompiler plugin (which requires IDA Pro) - also see idapro
  • Limited freeware version with restrictions available here
  • Plugins:
    • collabREate (open source)
    • CrowdRE
    • detpdb (open source)
    • Hex-Rays plugin contest winners (follow the links in the navigation pane, open source)
    • idados (open source), requires DOSBox
    • IDAPalace (open source)
    • IDA Toolbag (open source)
    • newgre.net plugins
• PE.Explorer

Libraries, frameworks and perhaps actual disassembler all in one

• BeaEngine (open source, LGPL license)
• distorm64 (open source, BSD license) and distorm3 (open source, GPLv3 and commercial)
• libdisasm (open source)
• libudis86 and udcli (open source, alternative site)
• miasm (open source)
• radare2 (open source)

Hex editors

• 010 Editor
• beye (open source)
• frhed (open source)
• Hex Editor Neo (freeware and commercial versions available)
• Hexplorer (open source)
• Hex Workshop
• HT Editor (open source)
• wxHexEditor (open source)

... anything not fitting the other categories

• CFF Explorer (freeware)
• DynamoRIO (open source)
• file(1) (open source), comes with most unixoid operating systems or can be built on them, based on libmagic(3), Windows version here
• firmware-mod-kit and binwalk (also included in the former, both open source)
• hachoir
• ldd(1) (open source)
• N-CodeHook (open source)
• N-InjectLib (open source)
• nm(1) (open source)
• PaiMei (open source)
• PE Explorer
• PEiD (and some signatures)
• PIN framework
• ssdeep (fuzzy hashes, open source)
• strings(1) (open source), comes with most unixoid operating systems or can be built on them, alternative Windows version here (not open source!)
• TrID file identifier (free for personal / non commercial use)
• YARA (open source)