Easta is a Napster-inspired peer to peer filesharing system, built to operate entirely over email (POP/IMAP+SMTP). More people have access to email than any other service, and in addition it gives easy access to file-sharing for the 90% of the planet stuck behind a firewall without a SOCKS connection. There's some nice side-effects too, such as asynchronous downloading, even when you're offline.
