Shared Projects are a new feature of Visual Studio 2013 Update 2. It was initially created to support universal apps apps for both Windows Phone RT and Windows RT, and that’s what most people know about it.
However there is also this genius Visual Studio extension that allows Shared Projects on any .NET project. It means that you can create a project shproj that contains a list of C# files. This file can be referenced by any project and will be included at compile time.
With Shared Projects you are always able to debug through any references code. This makes it very easy to find and fix issues or test new features.
Note that in Visual Studio 2015, this is an official feature: Shared Project : An Impressive Feature of Visual Studio 2015 Preview.
Thanks Matthijs ter Woord for noticing that.
via: The unknown beauty of shared projects in .NETGeert van Horrik.