For an existing website and the users in it,how to integrate a chat application like yahoo or gmail or any other with minimum code changes.
Ajax IM looks very good for this.
Alternatively, maybe something like PHP121?
These are both instant messenger style chat systems. If you're looking for something more like a chatroom, I have used Ajax Chat successfully in the past.
I think you'll need a fair bit of code changes however you approach this, if you want to support your existing groups of users.