Surely everyone knows about Adium by now?

And if not, perhaps you just aren’t using instant messaging enough to warrant looking for better solutions.
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Adium - “Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more”

There is a great group of screencasts to get you going with the app - http://adium.im/screencasts/ as well as an amazing number of xtras to allow it to do an amazing number of things for you - http://www.adiumxtras.com/

To read about the latest and greatest doings re: Adium check out their blog - http://adium.im/blog/

(and just to identify myself on this issues -- I pronounce it Ay-dee-um, like stadium... and when Leo Laporte says Add-ee-um it’s like fingernails on a chalkboard to me) Winking