Sometimes it's not acceptable to store data on external server without root on it. For these kind of uses Unison (mentioned in previous comments) is decent solution, but also AeroFS (aerofs.com), which works much smoother (less collision problems).

Anyway syncing data on several devices must became standard soon.

AeroFS also has a Windows port that works, too. I had no end of trouble with the Unison port.

Does anyone know how sparkleshare (http://sparkleshare.org/) compares?

Tried it a couple of weeks ago and could not get it to sync between a couple of Mac machines. Dropped the experiment soon after.

and DHT for instant uploading.