There are many others. You might try a Google or Yahoo! search for "Multi-Platform Instant Messenger" and see what you get. Here are my suggestions:
First, I would suggest Pidgin, which used to be Gaim. It is an extremely popular Instant Messenger, its customizable, and on top of all that, it's Open Source. I've used Pidgin and, personally, it's my favorite client to date.
http://www.pidgin.im
There's also Digsby. While I haven't actually used Digsby yet, it seems that it's quite the hit. It implements Instant Messaging platforms, like AIM, MSN, etc., while also connecting users via social networking, such as MySpace and Facebook. Be aware, however, that Digsby is still a newer program and may still have a couple of bugs.
http://www.digsby.com
Also, there's an article on lifehacker that compares five popular Instant Messaging programs. Pidgin and Digsby are included in the list, but you still might want to check it out.
http://lifehacker.com/375391/five-best-instant-messengers