Portable Library Tools Add-in for Visual Studio 2010

A new tool that enables you to create a single assembly shat can be shared by multiple .NET runtime environments, namely Silverlight, Windows Phone 7 and XBOX Gaming. Of course, you can target the desktop .NET runtime also when building class libraries but since the former ones include only a subset of the desktop .NET runtime, you only get the common denominator assemblies.

Download | MSDN Documentation