Linkinus is the preferred IRC client for a lot of our readers, I know (personally I tend towards Colloquy, but only because I'm a cheapskate and it's donationware), and they kindly dropped us a note to let us know they'd updated to version 1.3. The new build includes the ability to split chat windows (as seen above), in addition to a revamped UI, "major" performance upgrades, plaintext and Spotlight support for logging, and a whole slew of fun things to play around with, including, we're told, a "Whisper" style that will actually embed images, audio files, and YouTube and Google videos right into your chat windows (a la Campfire). Apparently IRC isn't just text anymore.
In fact, at $20 for all these features, Linkinus really is a steal for anyone who spends any amount of time in an IRC channel. Colloquy still does everything I need it to when I jump into IRC periodically, but if you want to chat like a pro in there, Linkinus is a great choice.