Just a few months after open sourcing of ASP.NET MVC 4, ASP.NET Web API, and Razor Microsoft is now happy to announce we will release the source code for the Entity Framework under an open source license as well!
Just like the open source ASP.NET components and open source Azure SDKs, Entity Framework will continue to have the same full-time Microsoft engineers working on the project and will remain a fully supported Microsoft product that will ship both standalone AND with Visual Studio.
The code being open sourced includes the Entity Framework Runtime and NuGet packages, Code First, the DbContext API, the Entity Framework Power Tools as well as pieces that shipped originally with the .NET Framework.
Entity Framework will live in a Git repository at http://entityframework.codeplex.com and you're welcome to follow, fork, discuss and file bugs against it as you see fit.
See more detail at Scott Hanselman’s blog Entity Framework Magic Unicorn (and much more!) is now open source with take backs