Status: Active development.

Link: http://colloquy.info/

Pros: Active development, a small amount of (sometimes quite pleasant) extras, advanced status, easily customisable.

Cons: Could do with a few more extras, toolbar and drawer icons are not that nice to behold.

Review: The kingpin IRC client for Mac OS X, Colloquy is easy to use and powerful. Development is active and dynamic. You certainly can't go wrong here!