IRC is one of the oldest chat mediums, and I'd be more than willing to bet that they'll surpass any other mediums. It's a solid foundation, especially since you can choose servers that cater to your needs (Freenode, my suggested server, has a long line focus on technology); BUT, they have their own services (as does any other IRC chat).
If you want to use something you can modify and lock down, there's always the shoutbox.
