Versions:

Json.NET comes in different versions for the various .NET frameworks.

-DotNet:
  .NET latest (4.0)

-DotNet35:
  .NET 3.5 SP1, Mono

-DotNet20:
  .NET 2.0

-Silverlight:
  Silverlight 4.0

-WindowsPhone:
  Windows Phone 7

Microsoft stopped support for the Compact Framework in Visual Studio 2010.
For a Compact Framework 3.5 build download Json.NET 3.5.

For a Silverlight 3.0 build down download Json.NET 3.5.