DotNetOpenID
This C# OpenID library adds OpenID 2.0 Provider and Relying Party support to your web site both programmatically and through convenient drop-in ASP.NET controls. Features * Add support for your site visitors to login with their OpenIDs by just dropping an ASP.NET control onto your page. It's that easy. An AJAX-style login control is also included for a slick, streamlined user experience. * Give your site members their own OpenIDs with the provider support included in this library. * Sample relying party and provider web sites show you just how to do it. * Easy access to all the functionality so you can customize how OpenID will operate on your site, whether you use ASP.NET or not. * Full support for custom extensions. Plus built-in support for Simple Registration, Attribute Exchange and PAPE. * Works in partial trusted shared hosting environments. * Support for web farms where state persistence, front-facing web servers and ASP.NET may not be standard or even available. * OpenId 2.0 and 1.x support. * Superior support for multi-byte Unicode identifiers. * Lots of Security features * 250+ unit tests to verify correctness.
Go to http://code.google.com/p/dotnetopenid/
See also these related projects.