With the new Messenger Connect Sharing badge you can now allow your users to easily share content from your site with their Windows Live friends. This functionality is open to all web sites now; you do not need to be in the beta program. Messenger Connect, is a set of development tools that allow you to bring Windows Live Messenger functionality into your website.
UPDATE 15th Oct 2010: Messenger Connect has now gone final.

There are two types of content that you can share with Messenger Connect:
- Article : This will include a page summary containing a thumbnail, page title, and description.
- Video : This will provide in-line playback of the shared video from within the page summary.
Microsoft has created two versions of the button with white and orange backgrounds.
Microsoft suggests that you the use the white background in order to create consistency across sites as well as teach the user what to look for visually in terms of Messenger Connect. If you strongly prefer orange, given the look and feel of your site, then you are welcome to use that instead.
Links: How to Implement Messenger Connect Sharing | Windows Live Messenger Connect Branding Guidelines.
