OpenID makes it very convenient to log in to many websites. You don't need to either use the same password (bad!) or use different passwords for different sites (hard!) anymore. You just log in with one simple URL, which is your URL. The Creative Commons wiki supports logging in through OpenID.
OpenID is an open, decentralized, free framework for user-centric digital identity.
OpenID starts with the concept that anyone can identify themselves on the Internet the same way websites do-with a URI (also called a URL or web address). Since URIs are at the very core of Web architecture, they provide a solid foundation for user-centric identity.
The first piece of the OpenID framework is authentication -- how you prove ownership of a URI. Today, websites require usernames and passwords to login, which means that many people use the same password everywhere. With OpenID Authentication (see specs), your username is your URI, and your password (or other credentials) stays safely stored on your OpenID Provider (which you can run yourself, or use a third-party identity provider).
How to use OpenID ?
You can watch two nice screencasts that explain it very well :
One from Simon Willison and the other one from Don McAllister.
