As the next step in fighting the Yahoos and Microsofts of the world, Google have released their own IM solution, Google Talk
Apparently it includes voice chat capabilites as well as traditional IM.
The client is currently windows-only, but as Google have chosen to implement Jabber, there's plenty of ways for everyone to connect. Google even provide a list of other applications that will work, and walkthroughs showing how to set them up.
Unfortunately, I've not been able to log in as yet - talk.google.com isn't responding to me on port 4223 :(
Full marks to Google for using something that's open, and for being so forward about listing 'unofficial' clients that will work.