has become the standard for web site login, taking the authentication off your site and centralizing it using OpenID enabled accounts they use every day.
OpenID makes it easier for users to login to your web site. Local account registration and login probably won't go away for some time, but login using OpenID providers such as Google and Twitter will dominate.
Google has a sample site that shows how a website can migrate users away from local logins, and have them leverage a centralized identity provider. The sample site provides common examples of OpenID registration and login in a very familiar product driven web site.
You can play with the site at openidsamplestore.com
, and they provide a list of OpenID frequently asked questions