Another Skype wannabe: Gizmo: A free phone for your computer:
Gizmo is a Free Phone for Your Computer
That makes calling as easy as instant messaging
An internet telephone. As simple as an instant messenger. Now you’re talking.
Make all your calls from the comfort of your desktop. With Gizmo, it’s point, click, talk. For free.
Say goodbye to high price calling, and say “hello” to anyone online, anywhere on earth.
Works on Macs, Windows and Linux.