The Windows Live ID service (formerly known as the Passport Network) is an identity and authentication system that you can use with your own Web site or client application:
- Web Authentication: Windows Live ID Web Authentication enables you to authenticate users on your Web site and offer personalized access to your site's content to any of the millions of registered Windows Live ID users. Download the Windows Live ID Web Authentication 1.0 SDK
- Client Authentication: Windows Live ID Client Authentication makes it possible for you to develop identity-aware client applications. Download the Windows Live ID Client 1.0 SDK Alpha Refresh
More info: Windows Live ID Home Page, MSDN - Introduction to Windows Live ID