I have a pair of USB external disks, one of which is at home, the other in my desk drawer at work. About twice a month, I use rdiff-backup to push a backup to the disk at home, then I take it to work and swap it for the one there.

I only have about 500GB that needs to be backed up, so I'm not that fussed about space (yet), and the twice a month incremental rarely takes more than an hour.

At some point, I'll see if I can persuade the in-laws to let me put a server and external disk on the end of their broadband, but it's not that urgent.