The Open Wengo project, a p2p-based system (like Skype), but open source (using the GPL License), is here at http://www.openwengo.com/
" OpenWengo is a community of enthusiasts and developers, creating free software products related to communication over IP. The flagship product of the OpenWengo project is a softphone which allows you to make free PC to PC video and voice calls, and to integrate all your IM contacts in one place.
OpenWengo was started and is supported by the french VoIP provider Wengo ."
Another open source alternative to Skype is Project Gizmo.