Techcrunch announced few hours ago that the Flash-based Yahoo! Web Messenger will be launched today. You can chat with both Yahoo! and Windows Live Messenger friends because of the integration with the Microsoft's messenger. It doesn't have all the features of the desktop Yahoo! Messenger but it's looking very promising to me.

To start the web instant messenger go to http://webmessenger.yahoo.com or http://web.im.



More web based instant messengers are listed here.