OpenID allows you to authenticate yourself to sites that support OpenID without having to remember or maintain dozens of separate logins. It’s basically meant for blogging sites that require authentication for leaving comments (such as LiveJournal), but should be made to work on any site.
The main advantages over other methods, such as Microsoft’s Passport, are that it is open, distributed and not under the control of a single entity.
Doesn’t look like it will be easy to setup on your own site, but I’ll give it a try. These days, I let Firefox store all my authentication data, but when using different systems or after system cleanups, I have to try to find out what username/password I had entered which is tedious and time-consuming. There really is a need for a simpler, more global mechanism and OpenID might be it.