Stumbled upon this thread today and thought I'd contribute this new cross-platform OSS diff tool that supports file and directory comparision. It is a good alternative to WinMerge for Mac. http://meldmerge.org/
Meld is a visual diff and merge tool targeted at developers. Meld helps you compare files, directories, and version controlled projects. It provides two- and three-way comparison of both files and directories, and has support for many popular version control systems.
Meld helps you review code changes and understand patches. It might even help you to figure out what is going on in that merge you keep avoiding.
