Google Talk

Google's Instant Message product hit the "shelves" earlier this morning for Windows only. There's no Linux or Mac client yet, but there are guides to connecting to the protocol on iChat, Gaim, and Trillian.

Neowin post:

Google Talk enables you to call or send instant messages to your friends for free–anytime, anywhere in the world. Google Talk is simple and intuitive to use. There's no clutter, no pop-ups, no unnecessary software to install. Talk also features some interesting VoIP features like Skype. Google Talk uses the Jabber protocol however instant messaging members of other clients doesn'’t appear to be supported.

So far, I love it! It's very lightweight and includes a newer GMail Notifier. Looks like you need to have a GMail account to use it, though. If you need a GMail invite to try the IM, I've got plenty - just send me your email address!