Hex Fiend is designed to work efficiently with large amounts (64 bits worth) of data. As such, it can work with arbitrarily large files without reading the entire file into memory. This includes insertions, deletions, and in-place editing. Hex Fiend can also efficiently save such changes back to the file, without requiring any additional temporary disk space.
Hex Fiend has a clean separation between the model, view, and controller layers. The model layer allows for efficient manipulation of raw data of mixed sources, making it useful for tools that need to work with large files.
Both the framework and the app are open source under a BSD-style license. In summary, you may use Hex Fiend in any project as long as you include the copyright notice somewhere in the documentation.http://ridiculousfish.com/hexfiend/
Hex Fiend comes with some sample code ("HexFiendling"), distributed as part of the project. And of course the Hex Fiend application itself is open source, acting as a more sophisticated sample code.