Got multiple Macs and don't feel the need to spend money on .Mac? Unison to the rescue! This little tool lets you synchronize directory trees and acts like its some kind of simple CVS, meaning it detects conflicts (for example when a file has been changed on multiple Macs) and lets you resolve them.

And did I mention it also does backups? Although you probably shouldn't use it as a backup tool. The backup functionality is more like a extra safety measure to make sure you don't overwrite your precious files with versions 3 years old.