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
Better CMS is a publishing focused and developer friendly .NET Open Source CMS.
A compositional, convention-based framework for complex web applications.
Thoughtfully architected, obscenely fast, thoroughly enjoyable web services for all.
NHibernate is a mature, open source object-relational mapper for the .NET framework. It's actively developed, fully featured and used in thousands of successful projects.
Lightweight, low-ceremony, framework for building HTTP based services on .Net and Mono.
A lightweight, object-oriented (Model-View-Handler) framework for modern web development in .NET.
Frictionless development for ASP.NET MVC single page web apps. Prototypical and dynamic capabilities brought to C#.
OpenRasta is an open-source .NET framework for building everything web, from web sites to RESTful APIs
The source code for ASP.NET MVC, ASP.NET Web API and ASP.NET Web Pages (aka Razor) are available under an open source license. Developers outside of Microsoft may submit patches and code contributions that the Microsoft development team will review for potential inclusion. These frameworks continue to be fully supported and staffed by Microsoft.
Orchard is an Open Source Content Management System built on ASP.NET MVC, nHibernate and Autofac.
Write regular CSS with your .NET apps, then add a few variables, mixins and nested rules. “.less” (pronounced dot-less) is a .NET port of the funky ruby LESS library.
At its core Glimpse allows you to debug your web site or web service right in the browser. Glimpse allows you to "Glimpse" into what's going on in your web server. In other words what Firebug is to debugging your client side code, Glimpse is to debugging your server within the client.
DotNetOpenAuth is a well-established open source library that bring OpenID, OAuth, and ICard capabilities to the Microsoft .NET Framework.
ELMAH (Error Logging Modules and Handlers) is an application-wide error logging facility that is completely pluggable. It can be dynamically added to a running ASP.NET web application, or even all ASP.NET web applications on a machine, without any need for re-compilation or re-deployment.
A simple but effective mini-profiler for ASP.NET MVC and ASP.NET. It does not attach itself to every single method call. Instead, it provides: an ADO.NET profiler and a pragmatic Step instrumentation that you can add to code you want to explicitly profile.
Massive is a "wrapper" for your DB tables and uses System.Dynamic extensively. Drop the code file into your app and change it as you wish. It’s a micro-ORM that hides some things and embraces others.
Dapper is a single file you can drop in to your project that will extend your IDbConnection interface. It’s a “micro-ORM” for people who love SQL.
N2 is a lightweight CMS framework. With just a few strokes of your keyboard a strongly typed model emerges complete with a management UI.
MvcMailer is an ASP.NET MVC Mailer, inspired by Ruby on Rails ActionMailer, that helps composing Email body using regular ASP.NET MVC views.
Designed to add functionality and ease-of-use to the ASP.NET MVC Framework, MVC Contrib is useful for developers looking to develop and test UI elements on top of the ASP.NET MVC framework.
Adds EditorTemplates for the new HTML5 input types (including email, tel, and url) to ASP.NET MVC 3 projects.
A T4 template for ASP.NET MVC
NuGet.org has an extensive directory of open source libraries which you can easily add to your applications via the NuGet package management system.
Wijmo Open is a kit of widgets built on jQueryUI and released to the community. Wijmo Open is open source and is free to use under either the MIT or GPL licenses.