IRC was one of the first things that got me excited about the Internet - I was studying Italian at the university, and realized that I could actually communicate, in real time with people on the other side of the planet, for free, back in 1993. I was utterly amazed by this. At first, I figured the people in the chat room (#italia) were BS'ing me and must just be Italians that lived in the US or something. A few years later, I managed to meet several of them in person when I moved to Italy for the first time.
Today, among others, I hang out on #startups on Freenode. It's not a very "serious" channel like some of the programming language ones are, but it's fun and interesting sometimes.