Meebo, instant messaging eveywhere. You can connect to MSN Messenger, Yahoo!, Myspace, Aim, without a chat software installed on your computer!
Meebo offers most of the features that your desktop application does, without requiring you to download any extra software. Meebo provides access to most IM networks you're likely to need, and lets you start using it within seconds of firing up the Web page.
Yes! Chat without software installed on any of the major networks: AIM, ICQ, Yahoo! Messenger, Jabber, GTalk, or MSN. Nice, clean interface and very fast.
Link: Free Chat Online with Meebo