I'm using the Dropbox Sync for playing the same World on three different PCs, one Linux-machine, two Windows7-machines. All machines have their "saves"-Directory linked into the Dropbox. As long as you don't run Minecraft simultaneously on more than one of these machines, it works pretty well :).
The initial sync is a bit slow because of the many files, and you should turn off the Dropbox Tray Notifications (because they become annoying with so many filechanges). Also, i am using the 0.8.107-Version of Dropbox, which allows syncing symbolic links and selective syncing. I'm not syncing my whole Dropbox on these machines, only the Minecraft-stuff. I'm willing to write an own guide for this, if someone cannot adapt the "Saved Data Dropbox Guide" to my solution.