I've been pleased with the DragonRuby Game Toolkit 'server', which has 12 channels in a category called 'Open Spaces' named after various video game characters.

I've been pleased with the DragonRuby Game Toolkit 'server', which has 12 channels in a category called 'Open Spaces' named after various video game characters. The community uses them freely and naturally. It gives a means to talk about a niche / ad-hoc stuff without stepping on anyone's toes. If you have a project going, you can effectively claim one of the unused channels for a while by simply posting in it. Because everyone is present without explicitly joining, the conversations are more discoverable and inviting, without having to nag anyone to do anything.