VoIP software

Asterisk PBX
The popular Linux-based open source software PBX switch. asterisk.org
Gizmo
A freeware VoIP client using SIP with Jabber protocol support. gizmoproject.com
GnomeMeeting
The popular Linux-based open source softphone, supports H.323 and soon SIP. gnomemeeting.org
Google Talk
A free VoIP system from Google. google.com/talk
Jajah
A freeware VoIP client with free videotelephony, chat, text messaging, voicemailbox and is compatible to SIP, Skype, Gizmo and IAX/H.323. jajah.com
OpenWengo
Voip-application published under GNU GPL license. openwengo.com
PeerMe
A proprietary freeware VoIP system which uses a messenger-like client. peerme.com
PhoneGaim
A free VoIP system based on Gaim and SIP. phonegaim.com
sipX
The popular open source SIP PBX, native SIP call control. pingtel.com
SJphone
SJphone is a popular free SIP/H.323 softphone that many services use. sjlabs.com
Skype
Skype is a free VoIP client that offers in and outbound PSTN facilities. It is not Open Source and is based on closed net-protocol. skype.com
Teamspeak
Voice communication programs popular in online gaming. teamspeak.com
Tivi
A SIP VoIP client softphone. tivi.com
YATE
A GPL (free) software VoIP telephony engine (VoIP server and client for H.323,IAX,SIP) for both Windows and Linux. yate.null.ro