While I was exploring Visual studio 11 and .net framework 4.5 BETA I found one very useful addition to it is Portable class Libraries.
There are many new thing is VS 11 and Framework 4.5, but this Portable class Library will be very useful. By building Portable .net libraries you can actually use them on multiple platforms without compiling them for different windows platform like Dotnet Framework, Compact Framework, Silverlight and Metro Style app Framework.
So now you can build you libraries with your own requirements and business logic and you can use them on different dotnet platforms.
For creating portable library project you have to install .net framework 4.5 beta and VS 11 Beta. After installing these you can create portable library project in C# or VB.net.
In Portable library project you can reference following Assemblies(these are by default referenced in template project)
For more details please see here