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

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

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

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