Trillian is a fully featured, stand-alone, skinnable chat client that supports AIM, ICQ, MSN, Yahoo Messenger, and IRC. I use Trillian and it is great to use one single client and communicate with all the other systems. Trillian is avaible at Cerulean Studios. There is a free version and a Pro version of Trillian.
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. I use a Jabber-client called Pandion. There are more clients listed at the Jabber website.
Google Talk uses the same concept as Jabber. There is a nice friend finder over at GTalk Profile.
Usually instant messaging is done using a software installed on your computer. meebo.com is a new alternative, you can use the big five (AIM, ICQ, MSN, Yahoo, Jabber/GTalk) from one platform online. I am using Meebo and it works nicely. Since I use Firefox as webbrowser it is easy to have Meebo running in a tab.
