Dropbox doesn't have completing, fixed interval backups and has fairly buggy, heavy, intrusive software (on linux). Google/Microsoft's services don't have differential backups and their software is generally lacking.
Rsync.net promises exactly what I want, but at 6/gb ($60/mo for 1 tb) it's 5x more expensive than Dropbox so I'm manually hacking around Dropbox's offering for the moment.