Blogroll for tag sip
Twinkle - A SIP softphone for Linux
Linphone - A SIP VoIP client for Linux
Ekiga - An open source VoIP and video conferencing application for GNOME
OpenWengo - A softphone which allows making free PC to PC video/voice calls, with IM, SMS support
SIPfoundry - A open source SIP based IP PBX, Nortel's product is also based on this
PJSIP - A open source embedded SIP and media stack
Can Ribbit Finally Bring Web & Voice Together? (GigaOM)
Ribbit - A new kind of "phone company" that provide a platform to build and use the service the way user/developer like it.
SIP Express Router/Media Server - A SIP server suite with free IP Telephony service
OpenSER SIP Server
Gizmo5 - A skype-like free internet calls service using standard protocol like SIP and XMPP