+1
I use Colloquy as well, and I like it.

I tried colloquy but the one thing i didn't like was that closing it disconnected from irc, while i know not everyone idles all day and its a minor annoyance to keep the window open in the dock. Well im currently using Linkinus its not free and its new and some features are still getting weeded in and bugs are getting weeded out i suggest to give it a try.
http://linkinus.com/

Why don't you just hide the program instead? Press command "Command+H" and its gone until you click back on the dock icon where it'll reappear. Alternatively option clicking (Alt+left click) on the desktop whilst in any app will accomplish the same thing and hide the currently running app.

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MacBook Pro 15" | Mac OS X 10.6 • 2 GHz Intel Core Duo • 2 GB DDR2-667 • ATI Radeon Mobility X1600 128 MB • 100 GB Serial-ATA
Hackintosh | Mac OS X 10.5.6 • 3 GHz AMD Athlon 64 X2 6000+ • MSI K9N AM2 • 2 GB DDR2-667 • NVIDIA GeForce 8800 GT 512 MB • ALC888 Audio • 300 GB Serial-ATA
iPhone 3G • 8 GB

@rogue

thanks for the tip O_o