Google Talk just went live.
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.
They provide a Windows client. It has a nice clean interface. It does not have advertising and annoying pop-ups plastered all over it.
Using their network requires a GMail account.
It uses Jabber/XMPP. Let's hear it for open standards.
It uses a custom protocol for the voice connection, which they claim they will fully document. They also plan to add support for SIP.
Update: I'm firstname.lastname@example.org.