I’m a fan of Adium, because it’s free/opensource and incorporates ICQ, Jabber, AIM, MSN, and Yahoo! IM. The highly/easily customizable interface is its biggest draw, for me.

For IRC, there’s no beating Colloquy, an opensource IRC/SILC/ICB client that supports plugins and looks great.

Adium w/Appearance Tweaks

Service Icons: Service Extensions White With Stroke
Status Icons: Twitterrific
Menu Bar Icons: Amora
Emoticons: NONE (I fucking hate emoticons)
Dock Icon: Flat Balloons 2.0
Color Theme: HUD
List Layout: HUD
Window Style: Group Bubbles
Automatic Sizing: Size to fit vertically; Width 128px
Messages: Modern Bubbling; Glass (Glass & Grey)

Colloquy