I like Xchat Aqua. (Takes a lot of customization, but it's a great client)
Other ones that people like:
Snak (Pretty similar to Xchat. Not free, though.)
Ircle (I don't like the interface, but if you do, it's a good client)
iRC (Never used it.)
Conversation (has the nice Panther look, but annoys me in that it doesn't allow you to use / commands)
I'll second the Xchat Aqua, but I like it after very little customization. (Just a couple of quick changes in preferences such as turning off indented nicks, and adding seconds to the time stamps.)
I've recently found a program called Minerva that is a bit better than X-chat; unfortunately, it's not free so I'll probably be switching back to X-chat when my trial runs out. Search on Macupdate for Minerva.
I just went back to try Conversation again.... and it sucks a lot less than it used to. About the only thing missing from it right now is multiple server support.... but at least it finally supports /commands. And the interface is spot-on.
I just went back to try Conversation again.... and it sucks a lot less than it used to. About the only thing missing from it right now is multiple server support.... but at least it finally supports /commands. And the interface is spot-on.