SIPNet is a p2p-sip framework, it support IM,audio,video based on p2p-sip protocols, and support file sharing like other p2p programs such as bittorrent. All SIPNet clients join to form a P2P overlay net and find each other using DHT algorithm.
GibPhone is a highly extensible VoIP/IM client for the .NETframework that uses a powerful plugin engine to allow for UI extensions and any call control stack / media payload / transport protocol combination.
Cabal is a C++ framework that provides support for high prestation parallel processing network servers and clients. It doesn't provide a protocol, it just gives you the abstraction of data sending/receiving in real time or priority scheduling.