An open source alternative to Skype is Project Gizmo, at http://www.theregister.co.uk/2005/07/05/project_gizmo_voip/
“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."
See the entry on Internet Telephony