CardSpace, nee InfoCard
There's not much reason to repeat what's at Wikipedia, so see http://www.wikipedia.org/wiki/Cardspace.
The significant thing about Cardspace as far as the Linux community is concerned is that at least in this area, Microsoft seems to have changed its tune. They now seem to be embracing the idea of interoperability among platforms and vendors. That includes the open-source community.
In the CardSpace area, Microsoft has made the specification available and they have said that they may be used on a royalty-free basis and without burdensome license restrictions. Although Microsoft won't be writing code that runs on Linux and other systems, they seem to be encouraging others to do so.
In short, Microsoft has eliminated all the barriers to implementation and deployment of the Cardspace identity metasystem.
http://www.identityblog.com (good starting place for more information)