Live Sync does a fair job of syncing folders in Windows (sorry) hosts. Whenever both computers have internet access, the folders sync. Live ID required (again, sorry.) Few limits on data (no files larger than 4GB, no more than 20,000 or so folders?)
Live Mesh is a similar Microsoft offering, but also give you web access to your files and stores a copy on the cloud. Sadly, there is a 5GB limit on total data.
A sine curve goes off to infinity, or at least the end of the blackboard. -- Prof. Steiner