Exposing a JSON API with ASP.NET MVC 3

This article on CodeProject shows one way (there are loads of ways, this is just one way) of how you might go about exposing a JSON API that is callable both from the web and via web based APIs from a standard desktop .NET client app, with the minimum of fuss, no config, other than the standard one you get with .NET installed.

It does however make use of ASP MVC 3 so that is assumed, as is .NET 4 and VS2010.