One should note that Dropbox also transfer your files without the cloud if you happens to have those devices within the same private network.

Also, as much as I think, having some Java programs running at the background is the best idea. I have already got a Java background service running (Crashplan) and it holds up to 500MB after running a longer time. I have yet to receive an invite (msg me if you've got one!), however a native client/background service/daemon would always be welcomed.