Microsoft Sites

CardSpace in MSDN Developer's Center
The starting point for Windows CardSpace information on MSDN.

Web Services Specifications 
CardSpace is built on WS-Security, WS-Trust, WS-MetadataExchange and friends. Go here to understand what's going on at the protocol level.

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.

External Sites

Kim Cameron's Identity Weblog
For those that dont know, Kim is the godfather of CardSpace, a.k.a. chief identity artchitect at Microsoft. On his blog you'll not only find some great conversations about CardSpace and the Identity Metasystem, but all things related to identity.

Wikipedia Entry
Fame at last: we have our own Wikipedia entry!

Open Source Identity Selector 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.