Windows CardSpace is one of the rich feature in .Net 3.0 which enables users to provide their digital identities in a familiar, secure and easy way. In the physical world we use business cards, credit cards and membership cards. Online with CardSpace we use a variety of virtual cards to identify ourselves, each retrieving data from an identity provider. Don't struggle with usernames and passwords, just choose an information card!
Following resources will provide you sufficient information on CardSpace including hands on lab and samples.
-- Naren