The `diff` UNIX utility has been a great tool for me. It performs a task I need to do regularly, find the differences between two files. Unfortunately, despite using it regularly, I am still routinely puzzled by its output. To the rescue: a tool I already had installed, but didn't know about.
FileMerge.app is an application bundled with Xcode that can be invoked at the command line to display differences between files (or directories!):
$ opendiff path1 path2
This opens a very intuitive GUI to display the differences. (Wow! And I thought piping `diff`'s output to TextMate to use its *diff mode* was an improvement...)
You can even [hook it into Subversion](http://ssel.vub.ac.be/ssel/internal:fmdiff). By the way, the IDL Workbench has a similar tool built into it.