ePost. A P2P email system without servers. Compatible with conventional email clients like Outlook Express, encrypted for extra security and rather free. Super interesting stuff!
ePOST works by storing data on all participant desktops. This means that you desktop is storing some of your mail, but mail for a number of other users, too. All of the data in encrypted, however, which prevents other users from reading your mail, and vice-versa. In order to survive a failure by a participating machine, ePOST replicas all data onto multiple machines, and makes sure that a suffient number of copies of each data item are stored at all times.