A simple and fast tool for browsing any file on your system, and making small hex-level changes.
Very very handy for verifying the contents of files you are writing out from your own applications without doing extra work yourself.
Also handy for snooping around in other people's files for ideas. You can also modify your hit points in Balders Gate, but anyway...
Note: Most of the source code is included with the application, select "View PeekIt Source" from the Window menu.