Something really, really cool.

What you can use Messenger Connect for

Messenger Connect enables three core scenarios for websites and app developers:

• Identity – makes it easy for users to sign in and sign up to your web site using their Windows Live ID
• Social distribution – lets users share the things they do on your website with their friends. Activities appear in Messenger, Hotmail, and across Windows Live properties, and other places Messenger social is displayed (including Windows Phone 7 and the very popular Windows Live Messenger iPhone app)
• Realtime shared experiences – lets users share an experience in real time with their friends

What’s new in Messenger Connect

Many of the components that have evolved into Messenger Connect have been around for several years (Messenger Web Toolkit, Live ID Web Authentication, Delegated Authentication, and the Windows Live Contacts API), but this is the first time we've delivered a suite of standards-based, self-service APIs as a package. To understand how Messenger Connect works, from authorization, to the different interfaces and controls, to the emerging standards/specifications we use (OAuth WRAP,  Portable Contacts,  ActivityStrea.ms, and  OData),  check out this post.

Very cool. Read on.