The Portable Libraries has features that are close to, yet different from another piece of functionality in Visual Studio: multitargeting. It is similar in that it will help you find the allowed subset of functionality in the .NET Framework given the combination of targets you select. It is different, because it will let you have a single project where you would normally have multiple projects for class libraries targeting .NET FX (versions 2.0 to 4.0, with full or client profile) and Silverlight. For Xbox 360 you have the Xbox 360 Game Library which comes close, but is a different thing altogether. PL projects are class libraries which can host reusable managed code over all of the previously mentioned.
You can install the tooling for PL from here. Once you have done that, a new library project is available:

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