Get into the Joy of Chat with JoyChat - A program developed by Jonathan Bell, based upon Open Mac! (Now with even MORE joy!)
It's written in Cocoa - so it's Mac OS X native, and ONLY runs in X
Here's the current (beta) version:
Beta 1 Build 4
Known problems:

• Possible security problem: In all previous builds, your full name was automatically entered for "Real Name" with build 3.5, this problem is corrected. It is reccomended that you upgrade if you care about this.
• Nickserv doesn't really work - Don't try to use the automatic register or expect automatic identify to work

If you come across any others, please email me.
Screenshots from Build 1!!!
Click the images to see full sized versions!