The Windows Live™ Messenger IM Control lets people on the Web reach you in Messenger by showing your Messenger status on your web site, blog, or social networking profile.  The IM Control runs in a browser and lets site visitors message you without installing Messenger first. The Windows Live™ Messenger Presence API is a JSON API which returns a users status & display name, this can be queried programmatically.

Use the wizard to generate the code you need to put on a web page. We'll be putting up more detailed reference docs in a few weeks. Until then, enjoy checking this out.

Also, check out Angus Logan's blog for more details.