Trillian is a Instant messaging client that allows you to simutaneously chat with just about all other chat clients. It is compatable with MSN, Yahoo, IRC, AIM as well as quite a few more that I don't remember. Its only draw back is that it doesn't have google talk/jabber support unless you purchase the pro version. How much does this client cost you ask? Absolutely nothing. Between the pro version and the free version, I couldn't notice any real differences, except for the google talk/jabber thing. Its awesome and it deserves to be checked out. Its home page is here.
This sofware is also downloadable here.