meebo now with facebook support 

www.meebo.com is a way to chat with all your contacts no matter which IM service they use - in your browser. And that without having to install dozens of software packages and applications. You can chat with ICQ, Gtalk, AIM, MSN, Yahoo etc... and now Facebook contacts in a single window (if you like)

Adium has had this for a while now, not in a browser though. I prefer Adium, cause it's tiny.

One more nice thing about meebo is the free widget you can install on your website or blog, so that your visitors can chat with you without having to know your ID,...

Oh and you can create Chatrooms (i tried an ipernity chatroom there some time ago)...