OpenH323 Project:
http://www.linuxtelephony.org/
http://www.opengatekeeper.org/
Vovida Networks, Inc. —Open Source Protocol Stacks (recently acquired by Cisco):
http://www.vovida.com/opensource.html
http://www.cs.columbia.edu/~hgs/rtp/
http://www.cs.columbia.edu/~hgs/sip/
http://www.ietf.org/html.charters/iptel-charter.html