Jitsi (formerly SIP Communicator) is a free and open source multiplatform voice (VoIP), videoconferencing and instant messaging application.

Linphone is a free voice over IP (VoIP) service and SIP client.

Asterisk is a open source communications project that lets you create telephony apps for IP PBXs, VoIP Gateways, etc.

Blink is the best real-time communications client using the SIP protocol.

Empathy is an instant messaging (IM) and voice over IP (VoIP) client which supports text, voice, video, file transfers, and inter-application communication ov…

Ekiga (GnomeMeeting) is a VoIP and video conferencing application for GNOME and Microsoft Windows.

Zoiper is an easy to use sip video softphone, with excellent voice quality and easy to setup.