OpenID is a digital identity system where users identities are defined by a URI. Its chief concern appears to remove the need for individual sites to maintain and provide disparate authentication methods (userid, password, etc). Instead, the various sites will authenticate with a trusted site that supports OpenID, called an Identity provider (IdP).
With Microsoft announcing support for OpenID within CardSpace I think we are going to see a heavy shift towards OpenID very soon, this is good for all of us. I personally have over 30 user names and passwords, or at least 30 that I can remember.
Anyway writing about this topic is not as useful as seeing it in action, so visit this screen cast by Simon Willison. There is also a Hanselminutes podcast that mentions this stuff but the screen cast is much more direct (Karl Franklin just interrupts far too much).
Here is a list of the IdP's I have found:
"Eighty percent of success is showing up." - Woody Allen