Yate is a next-generation telephony engine. While currently focused on Voice over Internet Protocol (VoIP) and PSTN, its power lies in its ability to be easily extended. It supports SIP, H.323, IAX, MGCP, Jingle, Jabber, E1, T1, analogic, robbed bit, ISDN PRI, BRI, and SS7. YateClient is an Instant Messenger and Voice application for Jabber/Jingle IM, SIP, H.323, and IAX VoIP protocols.
Ekiga (formely known as GnomeMeeting) is a soft phone, video conferencing, and instant messenger application for use over the Internet. It supports HD sound quality and video up to DVD size and quality. It is interoperable with many other standards compliant software, hardware, and service providers as it uses both of the major telephony standards, SIP and H.323.
Sofia-SIP is a SIP user agent library, compliant with the IETF RFC3261 specification. It can be used as a building block for SIP client software for uses such as VoIP, IM, and many other real-time and person-to-person communication services. The primary target platform is GNU/Linux. Sofia-SIP is based on a SIP stack developed at the Nokia Research Center.
WengoPhone is a multi-platform VOIP client sponsored and developed by WENGO and MBDSYS. The GUI part is Qt-based, and the Video-over-IP engine is based on the eXosip, oSIP, oRTP, and ffmpeg projects. The eXosip module is extended by a phApi module, which implements a high-level, easy-to-use call control API. WengoPhone supports PC-to-PC voice, video, and chat. One can use a standard SIP service provider such as Wengo to be assigned an incoming number, make calls to PSTN/cell phones, get voice mail, and more.
Gizmo Project is a program that uses advanced VoIP technology to allow you to talk with your friends around the world. Gizmo Project uses your Internet connection to make calls to people using other computers. It is simple, and allows you to talk clearly and for as long as you want.