There's an earlier similar product called GmailDrive, but this RoamDrive is more user-friendly (it's got around the maximum attachment size limitation). I'm now trying out the beta version -- it seems to basically work although still some bugs to iron out, and no easy way of using it for incremental backups.

I've got my own routines for doing this for the stuff on my various remote servers, but I'm never going to write a Windows version myself, so hope this improves soon ;-)