Years ago, Microsoft tried to be the guardian of our personal data with Microsoft Passport. Passport was conceived as a single sign-on for the internet. You were supposed to use Passport to log in into your bank's website, Ebay, the social networks you were member of... The intention was noble-minded in that respect that it allowed users to divulge their personal data selectively depending on the relationship with the diverse site owners.
But it didn't work, for reasons that Microsoft since then has acknowledged (see the "Seven Laws of Identity" by MS's digital identity guru Kim Cameron). Users did not grasp what a technology company had to do with the relationship with their banker or auction site (users probably even didn't like the idea of one centralised identity).
Windows Cardspace is built in into Vista and the latest version of the .NET framework. Unlike Passport, it is an enabler instead of an actor. It's a framework that provides a consistent interface for users to manage their multiple identities. On an auction site, you might want to have your bank assert that you can pay for the item you bid on. When commenting on a blogwith your own blog url, you want to have your blog software assert that you, the commenter, are really the person behind the url you pretend to represent. All of this becomes possible with the single, consistent user interface of Windows Cardspace, in which all kinds of identity providers can plug in.
Cardspace is an open specification (Windows Cardspace, previously called "InfoCard", being the MS implementation). Several parties are working on their own implementation, see for example this (cross-platform) Firefox plugin. It's a perfect illustration of Microsoft as "the company that helps you to organize your own information". As opposed to "the company that wants to organise all the world's information". Guess who that is...
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