I would like to add OpenId support to an app. It runs on ASP.NET MVC, and I would like to have it support OpenId authentication for users, and also allow users to use the app as an OpenId Provider. So basically I am looking for a good library that can be used as a provider and consumer for OpenId, and it has to be easily plugged into ASP.NET MVC (or at least easily wrapped by a controller and a set of views.
Stacked an Open Source implementation of StackOverflow uses DotNetOpenID which I think also probably SO itself uses...
Though it's built on WebControls paradigm so I think you'll have troubles implementing it with Microsoft MVC.
I know Grurrah uses a WebControl based Ajax library (Gaia Ajax - my previous startup) together with the Castle project MVC implementation, but I don't know anytig of the semantics. Though Grurrah is Open Source too, so I think you can easily browse the code of it to check it out :)
That means you can use DotNetOpenID together with Castle project MVC library (probably) and you can get to kick in Ra-Ajax for Ajax functionality (Disclaimer; I work with Ra-Ajax and also Stacked)
The MVCStorefront ASP.NET MVC sample implements OpenId and is pretty easy to follow. You can grab the lastest source code from here