XChat. It's totally free but you need ActivePerl to run scripts.

I use Colloquy for my MBP. While I used to use X-Chat Aqua I prefer everything on Colloquy more expect the Nick Auto Complete - It lists nick and you can sort by last spoke order which makes me have to type more of the nick.

On my PC I use X-Chat only because I used X-Chat Aqua on Mac so it made it easier. But I think mIRC is far better but the UI is very jumbled. But its great.

ON the note of X-Chat being free - on my PC after 30 days I had to buy a license but I know there are many versions.

On my Ubuntu machine I use Konversation and just love it I wish it was developed for Mac.

And Finally on PC I even go so far as to buy people mIRC Licenses for helping me out with computer things.
So in short,

On Mac - Colloquy
On Linux - Konversation
On PC - mIRC

While one of the builds of xchat (xchat 2) is free, it is also updated less than the official version as it uses the source code to compile builds. 2.8.6 is out in source, but the silverex still lists it as being 2.8.4. While I don't like the fact of paying for open source software, I'd rather pay 20 dollars for one that is updated regularly. Also if you want to run scripts, you would also need activepython and activetcl depending on if you wanted to run python or tcl scripts