Windows Live Messenger Library Beta Released

New: The Windows Live Messenger Library is a client-side library for building Web-based applications that integrate with Windows Live Messenger. With the Messenger Library, developers can build presence-enabled instant messaging applications that web site visitors can use to interact with each other as well as with Windows Live Messenger client users.

• The IM Control enables web sites to show the presence of Windows Live Messenger users and let site visitors engage in IM conversations with site owners or representatives.
  
• The Presence API is a an HTTP-based API that returns a Windows Live Messenger user's presence.

http://msdn2.microsoft.com/en-us/library/cc298458.aspx