JabberIM.exe will allow you to connect to all of the major IM systems from a single program. Trillian does this also. More information can be found here.

Quote:
Jabber is best known as "the Linux of instant messaging" -- an open, secure, ad-free alternative to consumer IM services like AIM, ICQ, MSN, and Yahoo (see the IM quickstart). Under the hood, Jabber is a set of streaming XML protocols and technologies that enable any two entities on the Internet to exchange messages, presence, and other structured information in close to real time. Jabber technologies offer several key advantages: