It was the beginning of 2005 when I discovered Skype; I liked it because it worked and it was multi-platform. Well, it was proprietary software, but there was nothing like Skype in the free software world.
Now there is Wengo.
Wengophone vs. Skype
Published
Aaron Fulkerson 
oops…maye not open source…
aaron – check out Gizmo too. Multi-platform, excellent quality, and open source! 🙂
http://www.gizmoproject.com/