Microsoft just released Windows Live ID Authentication SDK, allowing you to provide LiveID authentication to your website visitors. This will allow anyone with a LiveID account to instantly become a member of your website, even if you’re not using IIS+Windows.
LiveID is a unified login service, allowing users to log in many websites and services using only one account. MS uses it in Hotmail, XBOX Live, MSN Messenger Zune and others. Now Microsoft is allowing web site developers to use LiveID Auth, providing this SDK in a number of flavors: ASP.net/C#, Java, Perl, PHP, Python and Ruby.