ARS Technica recently wrote that Microsoft has opened up its Windows Live ID service to 3rd party web developers:
At first glance, Windows Live ID isn't doing much that is different from other online identity systems, such as the open-source OpenID or alternative systems used by Web companies like Google and Yahoo. However, Microsoft does have a few extra goodies to offer developers. Windows Live ID is not intended to replace the CardSpace identity manager that comes with Windows Vista, but rather to work with it. Tools such as CardSpace (as well as Novell and IBM's open-source Higgins project) manage identities for the user on the client side, while Windows Live ID handles things on the server side. The idea of opening up the API is to allow developers to tie the two together in a more seamless—and hopefully more secure—environment..