Microsoft Sites
CardSpace in MSDN Developer's Center
The starting point for Windows CardSpace information on MSDN.
Web Services Specifications and Open Specification Promise (OSP)
CardSpace is built on WS-Security, WS-Trust, WS-MetadataExchange and friends. Go here to understand what's going on at the protocol level.
BizTalk Services Identity Provider
A security token service (STS) that web sites and apps can use for authentication and access control. Apps and users can obtain secure authentication tokens from the Identity Provider Service. These tokens can then be exchanged, allowing communicating parties to provide proof of their identities.
Microsoft Live Labs Security Token Server
An online identity management service that lets you offload authentication functions, whether you are a user signing-in to web sites and services or a site or service owner who needs to authenticate users.
Kim Cameron's Identity Weblog
Kim is the chief identity artchitect at Microsoft Kim and the godfather of CardSpace. On his blog you'll not only find some great conversations about CardSpace and the Identity Metasystem, but all things related to identity.
Federated Identity Lab
This is Microsoft's website for the vendor interop tests. It acts as an test identity provider and a relying party.
Windows Live ID Information Card Management and Login
You no longer need to type in your Live ID username and password. You can associate an information card with Live ID and use that to login!
Non-Microsoft Sites
myOpenID
An Open ID provider with information card login for ease of use and phishing resistance.
Open Source Identity Selector (OSIS) Project
Open Source project to develop an open source identity selector which will target non-Windows operating systems.
Ping Identity - Ping Trust
Ping Identities Security Token Service that supports WS-Trust and will interoperate with CardSpace.
VeriSign Personal Identity Provider
PIP is an OpenID identity provider that provides information card authentication to mitigate phishing attacks
CardSpace for ASP.NET AJAX Control Toolkit
Control based on the ASP.NET AJAX Control Toolkit that adds CardSpace support to web pages easily
Wikipedia Entry
Fame at last: the Wikipedia entry!