MSN Web Messenger lets you talk online and in real-time with friends and family using just a web browser! You can Use it on any shared computer – at school, at work, at a friend’s house or anywhere you can’t install the MSN Messenger software.
But With new web-based instant messaging (IM) that is now available in Windows Live Hotmail worldwide, Microsoft is preparing to retire MSN Web Messenger service. The old MSN Web Messenger experience will end on June 30, 2009.
With Hotmail’s new web-based IM, you can chat from your Hotmail inbox or contact list, instead of going to MSN Web Messenger (http://webmessenger.msn.com/). Go directly to the Windows Live People page (also known as “your contact list”) at http://people.live.com and sign into Messenger (orange arrow in the picture below) to continue instant messaging on the web with your Messenger friends.
Instant messaging from Hotmail makes it easier to communicate and share in new ways in comparison to MSN Web Messenger.
Source: Windows Live Wire