You never mentioned any specific functionalities. I would assume that it would be a server side chat client with a operator login and guest uses for a live chat. What other features does your chat system need thats not covered by any of the systems that I posted?
I know there are a lot of Mac haters here but it's going to be tightly integrated with Mac OS X and iOS. It will be a sales chat system with server side requests, something like liveperson.com and it's going to integrate pretty tightly with my backend management system.
I've seen a lot of the open-source chat systems and most of them are like pidgin, I'm looking for a chat login, chat queue, the ability to moderate my agents, ban/allow users, etc. Does that make more sense?