We just announced http://referencesource-beta.microsoft.com

You now can:
• Debug into .NET framework source in Visual Studio (better than before)
• Download the source with .sln and project files, so you can open and browse in Visual Studio
• Browse the source online

For a tutorial, checkout our Channel 9 video:

Click here to play this video

 

For further info, see:
http://blogs.msdn.com/b/dotnet/archive/2014/02/24/a-new-look-for-net-reference-source.aspx
http://www.hanselman.com/blog/AnnouncingTheNewRoslynpoweredNETFrameworkReferenceSource.aspx