Colloquy is the best free, BUT, it's not exactly Liony as in the chat screen messes up if you 'flick' hard to scroll. And it has major bugs on initial configuration - when setting up my channels I always have to quit/restart it to have the changes take effect properly. And no this isn't isolated, I switch computers frequently and it is always like that ever since 10.4 when I started using it all the way up to 10.7.
It really needs a serious update but it kind of seems dead
I have always used it though despite its quirks