Trillian is a unique messenging program that allows you to sign onto multiple screen names from multiple messeger services with one program. last i use it, i was able to use Yahoo, MSN, AIM, ICQ, and IRC, i just visited their site, it says it works with facebook, allows you to use it in Internet explorer(Possibly others) without downloading...
http://www.trillian.im/ (http://www.trillian.im/)
+1, I enjoy this as well. There's a number of good, free options out there, Trillian just happened to find me first.