Microsoft continues its commitment to open source. We not only host projects at the CodePlex forge and support many open source projects, but we also create our own open-source projects that the community can improve and extend. In addition to Microsoft-supported open source projects, there are thousands of great open source projects, applications, and libraries that you can use with ASP.NET today. Here you'll find a few of our favorites. If you're looking for Samples, there's lots of samples here as well as on http://code.msdn.microsoft.com. We'll update these pages as we come upon other cool projects. - Jon Galloway and Scott Hanselman
WebAPIContrib is a collection of open source projects to help improve your work with ASP.NET Web API. We have divided the projects into a few different categories. For more information on each project, click through to visit the GitHub page.
Define your routes directly on actions and controllers in ASP.NETMVC and ASP.NET/Self-Hosted Web API.
DotNetOpenAuth is a well-established open source library that bring OpenID, OAuth, and ICard capabilities to the Microsoft .NET Framework.
Thoughtfully architected, obscenely fast, thoroughly enjoyable web services for all.
OpenRasta is an open-source .NET framework for building everything web, from web sites to RESTful APIs.
Lightweight, low-ceremony, framework for building HTTP based services on .Net and Mono.
Helper library for claims-based identity, access control and token based authentication for .NET 4.5, MVC, WCF and Web API.