Google Talk

They say talk is cheap. Google thinks it should be free. Google Talk enables you to call or send instant messages to your friends for
free–anytime, anywhere in the world. Google Talk offers you:
Choice: Get in touch how and when you want to–over email, IM or a call

Quality: Talk through your computer but hear your friends as if they were in the same room

Convenience: Your Gmail contacts are pre-loaded into Google Talk so inviting or talking to your friends is just a click
away

Google Talk is in beta and requires a Gmail username and password.