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.

CSipSimple is a Voice over Internet Protocol (VoIP) application for Google Android operating system using the Session Initiation Protocol (SIP).

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.

MicroSIP is a portable SIP softphone based on the PJSIP stack available for Microsoft Windows.

Voximplant helps to embed audio and video communication into your apps.

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.

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.

Phoner and PhonerLite are softphone applications for Windows operating systems available as freeware.