Another vote for iRC. Here's what I've tried:
1. Ircle - Used this back when I had OS 9. It worked, but it was a major kludge. Also it would do something freaky and annoying after I had it for 30 days. Like it would pop up a big annoying window every minute to get me to pay for the crap that was put before me.
2. AthenaIRC - Just as bad of a kludge for OS X. Lots of tiny meaningless floating windows.
3. JediKnight - Promising, looks like it has a very simple and easy to understand interface. Then I learned that it doesn't work with / statements, like "/me eats an orange" which basically defeats the purpose of IRC. At least it's free.
4. X-Chat Aqua - This is pretty good for one of those always-in-beta UNIX kludge applications. When I say always-in-beta I don't mean always in development like all open source applications... I mean it's always full of bugs and interface woes that are never resolved. Anyway, X-Chat Aqua is kind of strange and there are a few cases when only right-clicking can get you to certain functions (like adding a new preset server). See, just proof that having only one mouse button DOESN'T force software developers to design their applications for use with one button mice. Once you get into a groove with it, X-Chat is easier to use than the others, and it doesn't clutter up your screen with floating windows, but it's still fairly bad.
5. iRC - Finally someone got it right. This application has a very easy to understand interface, is all in cocoa (complete with customizable toolbar), and although it's shareware it's not "annoying" shareware. My favorite part of this app - BUTTONS for many different functions, such as kicking/op/deop if you're an operator, as well as file transfer (complete with a graphical file browser to select which file you want). I hate that IRC has the most confusing file transfer system EVAR, but this makes it simple. Get iRC if you want a good IRC client. None of the other ones even come close.
6. GAIM - Oh yeah, a bunch of the guys I know use GAIM. That's because they're Linux geeks who enjoy punishment and pain as long as all their software is in the early stages of alpha testing and has a penguin for an icon. I don't get it. Apparently, GAIM is a messaging client that can handle a number of different networks. But whenever I'm talking with them, they're always complaining that it can't read special characters (like parentheses or quotes) or it does this or it does that. Sounds like the worst piece of crap on the planet, and it's probably really confusing too. But I guess if you want to be a l337 1i/\\/uX h4XoR, you have to put up with that sort of crap.