What is OpenID

I signed up for a Ma.gnolia account the other day and, as part of the signup process, I was offered the opportunity to get an OpenID. I'm always a sucker for anything with open in the title so I clicked on the option and got an ID at myopenid.com. Although it looked interesting, a sort of open source equivalent of CardSpace (which I discussed briefly in a previous blog post), I had no clue how it was actually supposed to work until I read this excellent blog post by Simon Willison.