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

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

X-Lite is a freeware VoIP soft phone that uses the Session Initiation Protocol.

Telephone allows you to make phone calls over the Internet or your company’s network.

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

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

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