I found this site - reading through some stuff from the web 2.0 confernece media fall out.
Meebo allos you to IM betwen MSN, AOL, Yahoo, ICQ, Google Talk and Jabber.
No downloads or apps - web login and away you go. I've used it over the past week and love it -
http://wwwl.meebo.com/index-en.html
Cheers
John