IRC has been around forever, and I love it; there is nothing else like it.
If you are on Linux, I think IRSSI is the best client.
But you can still run IRC even if you're on Windows. There is mIRC, chatZilla, and HydraIRC.
On Mac, there are many cool clients such as Textual, LimeChat, or even emacs for Mac OS X (besides being a text editor, emacs does IRC!).
I was gonna include info on how to use IRC and a bunch of links, but really this site is all you need:
IRCHelp.org - anytime I google anything related to IRC, I just end up on that site. ;-)