I'm not sure I understand what OpenID is ... but I see plenty of sites that ask you for it. What is it? and how do I create one?
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OpenID is a Single Sign On service for the web
You have one login/password which can be used across all sites which support it.
This includes all StackExchange sites.
From OpenID Explained:
An OpenID is a way of identifying yourself no matter which web site you visit. It's like a driver's license for the entire Internet. But, it's even more than that because you can (if you want) associate information with your OpenID like your name and your e-mail address, and then you choose how much websites get to see about you. This means that websites that take advantage of OpenID won't bother you for the same information over and over again.
With the OpenID, you don't need to create a new username and password.
You just simply login to sites using your Google or Twitter or Facebook (or other OpenID providers) account. You can get an OpenID here
Here's the Wikipedia definition. There are a number of OpenID providers listed there as well. If you have any of the most popular webmail accounts available (Google, Yahoo, Hotmail), you've already got an OpenID.
The idea behind OpenID is that you only have one place where you need to log in. Other sites can redirect you, to your OpenID site and let you log in there, the OpenID site will then create a unique token, that lets the site know that you passed validation and optionally you can send some info along, like username, email etc.
There are several services where you can create an OpenID when you log in to this site the most popular are listed.
You could even create your own OpenID server if you wanted to. It's based on open standards and available for all to use.