MS released Windows Live ID Web/Client Authentication to public recently. Now 3rd party web sites can take advantages of millions of existed Windows Live ID users (previous Passport).

MS provides a language-independence ( sample include C# for asp.NET, Java, Ruby, PHP, Perl and Python. And once you read through the code, you can write your own Windows Live ID authenticate lib in your preferred language) , platform-neutral (Windows, *Nix, whatever), build-on-XML and strong encrypt algorithm(HMAC, SHA, etc) way to authenticate users.

It’s easy to use:

1. Login https://msm.live.com/app/ to get your unique APPID for your site.
2. Follow the samples, change your authentication.

It takes me ten minutes to setup and use.

The Windows Live ID will ONLY pass a unique anonymous user id (specified to your site) to the site. This seems to be a little useless because it even lacks email address. But, it just protect Windows Live ID user’s potential privacy.

Links:

MSDN

Windows Live ID Web Authentication 1.0 SDK