Google have launched Google Talk, a chat service that uses GMail logon/password for authentication, and supports instant messaging and voice.
It uses the XMPP protocol for instant messaging, so other clients can connect (including those on non-Windows platforms that their client doesn’t support yet), and they say they will support SIP in future for voice.
Now… why wasn’t this included with their Desktop sidebar? That would be one killer helper app. Not that I’m convinced the world needed another IM network.