I've used Campfire by 37Signals and thought it was pretty cool. A bit pricey for me though and I would only use it with friends so I want a free alternative. Found TalkerApp and they offer a do a free version but it only offers public chat rooms.
Google Chat and Skype would've been great but they don't offer persistent chat rooms ( as far as I know) and I would like the option to have the transcripts of conversations.