Depends on what you want a "peer" to be. A peer being an Free Software server, which anybody can host: Appleseed, Diaspora, Elgg, and many more. A peer being

Depends on what you want a "peer" to be. A peer being an Free Software server, which anybody can host: Appleseed, Diaspora, Elgg, and many more. A peer being like a file-sharing application running on each persons computer: XMPP would be a good start, but I don't know any projects, who have built something noteworthy on top. The big problems: Friends must be able to interact with me, while I'm offline. Browser integration for "share buttons".