pakled: When I said "I have heard" it was just a form of expression. I've used it.
shugo974: Google has also made it. If you have an account already on AOL, a blog on blogspot , an account on flickr, Yahoo, and various other sites you will already have an OpenID, which is an url.
For example an OpenID on blogspot is this: http://nameofyourblog.blogspot.com/
All blogs on blogspot are OpenID's.
The practical purpose is to only give a username and password to one site, for example a site that you trust most. Let's say I trust google most and thus I only give username and password when I login to Google. Although when I want to login to other sites I only give the http://nameofyourblog.blogspot.com/ as login, and after that, it either redirects me to the login site at google or if I have trusted that site, then I can check the remember checkbox.