This looks interesting:
DIBS, the Distributed Internet Backup System, a peer-to-peer system designed for data backup. Rather than sharing files, you share your surplus disk space for other users to store encrypted backups in; in return, they do the same for you.
Which sounds promising, though I can see some potential problems with reliably getting things back. (How many redundant copies of data are backed up? What happens when peers disappear from the network?) Perhaps what this needs is some sort of "heartbeat contract" mechanism; where peers agree that if they don't communicate for a period (say, a week, or perhaps a month), the other peer has disappeared, and its disk space can be reclaimed for new backup partners.
(via Slashdot)