Check out the free software Skype:
http://www.skype.com/
"Skype is the next phenomenon from the people who brought you KaZaA. Just like KaZaA, Skype uses P2P (peer-to-peer) technology to connect you to other users ? not to share files this time, but to talk for free with your friends."
I've used it, it works great, and it sure beats long distance phone bills.