Rob Conery did another excellent episode of his MVC Storefront series the other day where he ripped out his user membership code and replaced it with OpenId. Implementing the whole OpenId protocol yourself isn't necessary as there are some great libraries...
I've been monitoring the OpenID progress for a while now, and when I saw this post from Scott Hanselman about how easy it is to add OpenID support for ASP.NET MVC through the use of the DotNetOpenId package, I had to give it a try. It it sure wasn't hard...