Status: Active development.

Link: http://www.adium.im/

Pros: Active development, tons of (often high quality) extras, advanced status, extremely customisable.

Cons: The sheer number of extras can be daunting and can lead to a mis-match of theming.

Review: The jewel in the crown here is the extras site, which hosts countless style busting add-ons from message styles to status icons. Unfortunately, this can also be it's downfall as users are forced to swim through countless piles of rubbish (which I have contributed to) just to get the application to their liking. On the plus side, Adium supports zillions of different protocols, including IRC, something that has been sorely confined to single protocol apps for too long, sans Fire, which is now deceased.