Gizmo Project

I use Skype which looks to be pretty much the same thing.

Skype works well for me and lets me talk to my Dad in Japan for free when we're both on. Skype also has the ability (for a fee) to let you call real phones and to have a real phone number that people can call that will get routed to your computer.

I like it a lot but can't say that I use it that often.
Skype has some security issues, and is a closed protocol. This looks really interesting, but I don't use the phone enough to even bother trying this.