Adium ※

Adium is a multi-protocol Mac OS X IM client based on the core Gaim library that supports a wide range of services (from Jabber to MSN) and has a number of nice features, such as an infinitely themable interface, Growl integration and... a green duck.

Pluses:

• It understands about Fast User Switching and will reply "I have switched logged in users. Someone else may be using the computer." to your buddies, as well as setting you away.
• Growl support.
• Address Book integration - the killer feature.

Minuses:

• The duck can get on your nerves. I used to use the Milk icon and theme, but since there was no easy way to automatically update the buddy state icons upon each upgrade, I eventually gave up and just started using the "Visual Notifications" scheme (which put an end to the squawking)
• Changing MSN status/display name can be a chore.

Resouces:

• AdiumBook, a tool to help you update your Address Book with Adium details (IM contact, contact picture, etc.)
• Quicksilver plugin, source, forum thread. This is so mind-boggingly useful I have to wonder why it's not easier to find.