Unison File Synchronizer
I'm trying out Unison File Synchronizer as a way to keep my two machines at work in sync. Unison inherits many concepts from source code management tools like CVS and SVN, and can manage bidirectional updates (even merges). I've done a test sync, firing the contents of my Powerbook over on top of my deskop's home directory. It took only about 20 minutes or so to copy stuff over, but a long, tedious process of approving or reconciling conflicts made the process last many times longer than that.
