Generally speaking, Skype works pretty well for me.
The Gizmo Project is quite intriguing:
http://www.gizmoproject.com/
The software works well for a Beta. I live in hope that they might one day support cross platform video conferencing before Skype does... (iChat and AIM on a PC have been disappointing).
I have been able to place calls to normal phones with Gizmo easily and the quality is good.
Anyone care to share their experiences?