Diffract is a simple, free GUI diff/merge tool for MacOS X (and eventually others!).
I originally started this project because I noticed a distinct lack of good, free GUI diff/merge tools for Mac OS X. The ones I could find were either not very useful, or a bit expensive. There are some ported over from Linux, but they tend to be a pain to get working, and don't look so great either. So I decided to help remedy this problem.
Among other things, the tool should support both side-by-side and unified views, 3-way merges, in-line editing for merges, syntax highlighting, and custom font/color schemes.
The project is being developed primarily for MacOS X, but the intention is to make the base code as platform-independent as possible. This way, I can bridge in platform-specific GUI libraries, and make a nice-looking, native GUI version for the three major platforms. This is also a good excuse to learn a number of different GUI libraries for different platforms. However, if I can't quite accomplish this, it's going to stay a MacOS X exclusive.
Diffract is licensed under the GPL, and written in C++.