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:
This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2004/09/get-your-hex-on.html