It's kind of a specialized tool, but when you need it*, a hex editor is indispensible. I've used Hex Workshop since, geez, 1997! I recently purchased an upgrade to the latest version and I was pleased to see it's still under very active development, with lots of great new features:
One of the more interesting new features is the structure viewer. I have a zip file open in the above screenshot; notice the zip file structure in the bottom left, with the filename highlighted. You can actually do in-place editing on the structures, and of course, define your own. There's also a fairly sophisticated resynchronzing binary file comparison mode, too.
* I'm currently revisiting some MP3 ID3 tagging code; I need to look closely at the structure of the tags in the binary fine. Loverboy? Hey, I never said I had good taste in music.