I've already tagged Unison, but I find it so very useful that I just have to write this short blog post to praise it. If you work on two computers and want to keep a set of files in sync, you should try Unison. It's open-source, cross-platform, and the newest version works great on Mac OS X (fink's version was broken when I tried it).
In contrast to other syncing or backup programs, Unison is bidirectional. For example, I just edited some slides for a talk I'm giving tomorrow on my laptop, while I made some changes in the report on my main computer. Then I ran Unison, and it sent the files in the appropriate direction.
Minor gripe: I've tried to sync a huge folder with it and it failed (20 GB / 8600 files+folders), but it works fine and is fast for my normal syncing (350 MB / 1200 files+folders). However, in the first case I really wanted to make a backup, so I just used SuperDuper for this one folder.