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 solu

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.