With Gizmo5, not only can you use your PC to make or get phone calls on Linux, Windows, and Macintosh PCs. But unlike similar programs, such as Skype, Gizmo5 uses open standards like Session Initiation Protocol (SIP) and Jabber, which makes it interoperable with a variety of clients.
Previously known as the Gizmo Project, Gizmo5 is both the name of a Voice over Internet Protocol (VoIP) network (with its own servers and users, working over the Internet) and of a program that lets you communicate by using that network. Though it uses open standards, Gizmo5 doesn't qualify as open software itself. It uses several proprietary codecs, and the client code itself is closed source.