Unison File Synchronizer
I've gotta remember to blog these things when I come across them. I'd swear that I found this one on Ross' blog, but when I actually came to a point where I wanted to try it out, I was unable to locate it there. Perhaps it was on a blog that I found via Ross' blog. Who knows.
(Note to Ross: Add a good search facility to Blogware!)
Anyway, here's the link to the Unison File Synchronizer.