A lot of the chat apps have IRC support. I was part of a channel where the majority of people were using Adium (Mac) or Trillian (Windows). It was pretty easy to get people to join if they were already using one of these IM clients for AIM/MSN/etc. For people who don't already use a multiprotocol IM client like that, I often point them to Mibbit as a quick web-based way of getting on: http://mibbit.com/. But the people already using an IM client were more likely to become regulars.
