Uh-oh Skype, you went proprietary and no developers wanted to play with you. Welcome to the market Gizmo 
Gizmo – A free phone for your computer
From Slashdot:
valmont writes "The Register is offering an interesting introduction to Project Gizmo, a new player in the Voice over IP field, poised to challenge Skype with its ability to interoperate with others thanks to the SIP protocol it complies to. Whereas Skype has selectively licensed usage of an API that offers limited insight into a closed protocol, a closed ecosystem solely controlled by one organization, the SIP protocol is open. Free open-source proxy/server implementations are sprouting up, and many developers are actively working on SIP clients. The Gizmo Project is the first to bring a truly-usable, user-friendly, cross-platform SIP client (Mac, Windows, Linux coming soon) to market. Meanwhile, theappleblog.com is already offering a Gizmo Project Wish-List to promote better interoperability between current and upcoming SIP providers, to make it more practical for users of disparate SIP clients to communicate with one another."