OpenID's allow you to securely log into a new site with an existing username from another site so you don't have to keep setting up for new accounts every time you find a new site.
Several large organizations either issue or accept OpenIDs, including Google, Facebook, Yahoo!, Microsoft, AOL, MySpace, Sears, Universal Music Group, France Telecom, Novell, Sun, Telecom Italia, and many more.
This is how you sign in for some of the more popular accounts.
Blogger = blogname.blogspot.com
Google = http://www.google.com/profiles/username or Go to your google profile, look at the bottom of the page. It says your profile url is :
Yahoo = https://me.yahoo.com/username
Myspace = www.myspace.com/username
Wordpress = username.wordpress.com
AOL/AIM = openid.aol.com/screenname.
Flicker = Sign in with your Yahoo button or use your photostream url.
Orange = Sign in with Orange button or enter orange.fr
LiveJournal = username.livejournal.com
For those looking for Facebook, it appears they accept OpenID, but I do not see where they are issuing one yet. If they start I will update this.
You may choose to associate information with your OpenID that can be shared with the websites you visit, such as a name or email address. With OpenID, you control how much of that information is shared with the websites you visit. Your password is only given to your identity provider, and that provider then confirms your identity to the websites you visit. Other than your provider, no website ever sees your password, so you don’t need to worry about an unscrupulous or insecure website compromising your identity.