you got a binary file and don't know its contents. Or some software creates binary files where you have a specification but don't want to decode them manually.
Synalyze It! allows you to create a grammar for binary files interactively. Unlike in regular hex editors or viewers the files are interpreted automatically for you!
The grammars are stored as XML files and contain all the structures that may occur in a file of a certain format, just like XML schemas. It's even possible to inherit structures from others (like in OO languages) so you don't have to repeat for example a length field that appears at the beginning of each structure.
Essentially it's a modeling tool for arbitrary file formats.
Additionally Synalyze It! is a full-featured free Hex Editor allowing you to edit files of any size.
For some well-known formats you can download grammars here. Matching grammars are suggested automatically when you open a file.
Right now you can download Synalyze It! for Mac OS X (Leopard, Snow Leopard & Lion) and give it a try. In case of questions or problems please give me feedback. The latest changes can be tracked here; stay tuned also with Twitter.
If you like Synalyze It! consider buying the Pro version in the Mac App Store.
The Pro version has even more useful features.
See it in action on YouTube (Thanks, Aaron!)
Recent updates:
2012-04-20: Published grammar for Zelda Solarus DX saved games (Pascal)
2012-03-21: Added ACV grammar (thanks to Jonathan Wight)
2012-03-19: Added PVR grammar (thanks to Andreas Löw)
2012-02-22: Synalyze It! 1.2 released with many improvements
2012-02-22: Published grammar for Zj-Stream print files
2011-12-10: GraphViz export in Pro version 1.1.6 included
2011-11-19: Version 1.1.5 is now available
2011-11-07: Uploaded a grammar for CPF (Canon Preset Files) Thanks Chris!
2011-11-07: Uploaded grammars for Pokémon files - thanks, Kelvin!
2011-11-04: Published a grammar for ZIP files (needs version 1.1 - scripting)
2011-11-02: Synalyze It! Pro is available in the Mac App Store
2011-06-15: FLV files (Flash Video) can be decoded now
2011-05-13: A grammar for PCX image files is added
2011-04-14: A basic grammar for Audible files can be downloaded now
2011-04-08: A grammar for ICC (color) profiles is available now
2011-04-05: This is cool: Pascal Werz provided a grammar for Mach-O files.
2011-04-03: A grammar for saved games of Borderlands was added :)