Decompilers

• Debuggers:
  • OllyDbg, free, a fine debugger, for which you can find numerous user-made plugins and scripts to make it all the more useful.
  • WinDbg, free, a quite capable debugger by Microsoft. WinDbg is especially useful for looking at the Windows internals, since it knows more about the data structures than other debuggers.
  • SoftICE, SICE to friends. Commercial and development stopped in 2006. SoftICE is kind of a hardcore tool that runs beneath the operating system (and halts the whole system when invoked). SoftICE is still used by many professionals, although might be hard to obtain and might not work on some hardware (or software - namely, it will not work on Vista or NVIDIA gfx cards).
• Disassemblers:
  • IDA Pro, commercial, top of the line disassembler/debugger. Used by most professionals, like malware analysts etc. Costs quite a few bucks though.
  • W32Dasm, free, a bit dated but gets the job done. I believe W32Dasm is abandonware these days, and there are numerous user-created hacks to add some very useful functionality. You'll have to look around to find the best version.
• Decompilers:
  • Visual Basic: VB Decompiler, commercial, produces somewhat identifiable bytecode.
  • Delphi: DeDe, free, produces good quality source code.
  • C: HexRays, commercial, a plugin for IDA Pro by the same company. Produces great results but costs a big buck, and won't be sold to just anyone (or so I hear).
  • .NET(C#): dotPeek, free, decompiles .NET 1.0-4.5 assemblies to C#. Support for .dll, .exe, .zip, .vsix, .nupkg, and .winmd files.

Some related tools that might come handy in whatever it is you're doing are resource editors such asResourceHacker (free) and a good hex editor such as Hex Workshop (commercial).