[pascoej]

I've really come to enjoy Synalyze It! while developing some media format decoders. It has grammars for a ton of file formats builtin.

Well worth the 90$ for the pro version if you spend a lot of time in a hex editor.

[minimaul]

There's also 010 Editor, which has a very similar thing for creating file format grammars. Invaluable if you work with complex binary files regularly.

The scriptable functionality to decode a file format that this and Synalyze It do makes them my two favourite hex editors.

This hex editor looks like a great start, though :)