I think HipChat is hard to beat in this area. We've been testing it at work, and early thoughts are very positive. They have native (Adobe Air, but much better

I think HipChat is hard to beat in this area. We've been testing it at work, and early thoughts are very positive. They have native (Adobe Air, but much better than what I'm used to), mobile and web clients covered, some IRC-like commands and a cool @mention notification system.