As quoted from the OpenID Foundation’s Web Site…
OpenID eliminates the need for multiple usernames across different websites, without needing to create new passwords
You get to choose the OpenID Provider that best meets your needs and most importantly that you trust. At the same time, your OpenID can stay with you, no matter which Provider you move to. And best of all, the OpenID technology is not proprietary and is completely free.
If you use services, such as Google, Yahoo! and WordPress.com, you already have your own OpenID.
OpenID is a decentralized standard, meaning it is not controlled by any one website or service provider. You control how much personal information you choose to share with websites that accept OpenIDs, and multiple OpenIDs can be used for different websites or purposes. If your email (Google, Yahoo, AOL), photo stream (Flickr) or blog (Blogger, WordPress, LiveJournal) serves as your primary online presence, OpenID allows you to use that portable identity across the web.
OpenID in action