Microsoft has just announced via Scott Guthrie’s blog that they will be releasing ASP.NET Web API and ASP.NET Web Pages (aka Razor) as open source under the Apache license.  These products will join others that Microsoft has release as open source including ASP.NET MVC which has been open source since V1.  Perhaps more key to this announcement though is that Microsoft will for the first time be allowing external developers to contribute to the code base.

We will also for the first time allow developers outside of Microsoft to submit patches and code contributions that the Microsoft development team will review for potential inclusion in the products. We announced a similar open development approach with the Windows Azure SDK last December, and have found it to be a great way to build an even tighter feedback loop with developers – and ultimately deliver even better products as a result.  – Scott Guthrie’s Blog

Another key aspect is that while Microsoft will allow developers to contribute to the products, they will continue to be fully supported by Microsoft and will ship both stand-alone and with Visual Studio.

Scott Hanselman posted shortly after this announcement and answered a few questions including why ASP.NET WebForms is not going to be included.

The components that are being open sourced at this time are all components that are shipped independently of the core .NET framework, which means no OS components take dependencies on them. Web Forms is a part of System.Web.dll which parts of the Windows Server platform take a dependency on. Because of this dependency this code can’t easily be replaced with newer versions expect when updates to the .NET framework or the OS ships.  – Scott Hanselman’s Blog

The source code will be hosted on CodePlex at http://aspnetwebstack.codeplex.com.  Microsoft has recently added Git support in addition to TFS and Mercurial support.