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

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.

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

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

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.

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

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