We are releasing a new plugin today that allows WordPress users to add a live cross-site chat to the blog page/post or sidebar.
Chat is centralized, so people from one blog can chat with people from another blog. Communication takes place in chat rooms, which can be searched by tags.
Find out more about Geesee chat at www.geesee.com.
You can download the plugin from http://www.geesee.com/provide/plugins.aspx
Let me know how do you like it.