As we know OpenID is an open, decentralized, free framework for user-centric identity. The goal is to have the user remember one user name and password and login to any site which supports the protocol. It is light-weight and supports SSO.
In the past few days there has been a lot of traction to OpenID adoption with the announcement for its support from the giants AOL, Microsoft and Digg. So if you have an AOL account then your open id is http://openid.aol.com/username and you can login to any website that supports OpenID. With the growing number of startups the adoption rate for OpenID is poised to grow rapidly.
To start using OpenID you will need to obtain one. If you already have an AOL account you could use that if you like. Or you can obtain one from http://www.myopenid.com/. For a quick overview of OpenID here is a video.
From a developer perspective for you to support OpenID either as client or server there are several resources available as noted below:
OpenID Enabled - OpenID community
OpenID Client/Server Libraries - C#, Java, Perl, C++, PHP, Ruby, ColdFusion
BotBouncer - A CAPTCHA service for OpenID
I was looking at OpenID4Java API - Consumer side and is pretty straight forward.
OpenID and CardSpace
Here is a post explaining how these two complementary technologies can work together. Its interesting.