I've used Unison (http://www.cis.upenn.edu/~bcpierce/unison/) to get a dropbox type setup running across several machines.

I've thought about that as well. Could you give some details on the topology used? Did you run unison in automatic mode via cron?

I use unison for my code repositories (I use dropbox for other things) and I don't bother with it in cron. It's been fast enough in my experience that I don't mind waiting, and it sometimes (rarely) requires manual conflict resolution.

I use rsnapshot (also built on top of rsync/ssh) http://www.rsnapshot.org

I just use http://rdiff-backup.nongnu.org/