Just what I needed another messenger service, but here I am as smalljones (same as my Gmail address). I need buddies to test it out, so drop me a note there — not Gmail but Google Talk. It supports numerous clients including GAIM, iChat, and Psi. See the Google Talk pages for set up info. There is also a custom Google Talk client for Windows.
Note: The audio portion of GTalk only works with the official Google Talk client for Windows.
Rumor: Is Google Talk a prelude to Google buying Skype?
Note: iChat to iChat using the Google Talk server (not the client) supports audio chat.