I'm fairly happy with my strategy of keeping everything important in my CVS and Subversion repositories, but BackupPC looks like an interesting cross-platform solution.
For Mac users there's also, of course, Time Machine (which is reasonable enough in its way, but suffers from the usual "Macs don't like to sleep with USB devices plugged in" issues) and the gold standard: Carbon Copy Cloner. CCC is 'just' a front-end for rsync, but does a neat job.
A number of my faculty members have had good luck with Unison as well.