Via Many to Many, Earthlink has released SIPshare, a proof-of-concept peer-to-peer file sharing prototype using SIP (Session Initiation Protocol), designed originally for voice and video communications:

EarthLink believes an open Internet is a good Internet. An open Internet means users have full end-to-end connectivity to say to each other whatever it is they say, be that voice, video, or other data exchanges, without the help of mediating servers in the middle whenever possible. We believe that if peer-to-peer flourishes, the Internet flourishes. SIPshare helps spread the word that SIP is more than a powerful voice over IP enabler --- much more. SIP is a protocol that enables peer-to-peer in a standards-based way.