Linux

Asterisk	The open source PBX and multi-protocol VoIP server
sipX	The SIP PBX for Linux from SIPfoundry
OpenPBX	An open source PBX with support for OH323 PSTNGW
SIP Express Router (SER)	The open source SIP proxy server
sipXphone	sipXphone - The SIP softphone for Windows and Linux from SIPfoundry
sipXezPhone	A new SIP softphone from SIPfoundry
GnomeMeeting	The open source H.323 video softphone
Skype	A proprietary protocol VOIP system built using Peer-to-peer (P2P) techniques.
Linphone	The open source SIP softphone for Linux
KPhone	KDE based Softphone for SIP protocol
PhoneGaim	Gaim instant messaging client merged together with a Linux SIP softphone.
Twinkle	QT sipphone

See Also