The Json.NET library makes working with JavaScript and JSON formatted data in .NET simple. Quickly read and write JSON using the JsonReader and JsonWriter or serialize your .NET objects with a single method call using the JsonSerializer. 

• LINQ to JSON
• Lightning fast JsonReader and JsonWriter
• The JsonSerializer for quickly converting your .NET objects to JSON and back again
• Json.NET can optionally produce well formatted, indented JSON for debugging or display
• Attributes like JsonIgnore and JsonProperty can be added to a class to customize how a class is serialized
• Ability to convert JSON to and from XML

Example

Product product = new Product();

product.Name = "Apple";

product.Expiry = new DateTime(2008, 12, 28);

product.Price = 3.99M;

product.Sizes = new string[] { "Small", "Medium", "Large" };

string json = JavaScriptConvert.SerializeObject(product);

//{

//  "Name": "Apple",

//  "Expiry": new Date(1230422400000),

//  "Price": 3.99,

//  "Sizes": [

//    "Small",

//    "Medium",

//    "Large"

//  ]

//}

Product deserializedProduct = JavaScriptConvert.DeserializeObject<Product>(json);

Documentation

Json.NET - Quick Starts & API Documentation

History 

Json.NET grew out of projects I was working on in late 2005 involving JavaScript, AJAX and .NET. At the time there were no libraries for working with JavaScript in .NET so I began to grow my own.

Starting out as a couple of static methods for escaping JavaScript strings, Json.NET evolved as features were added. To add support for reading JSON a major refactor was required and Json.NET will split into the three major classes it still uses today, JsonReader, JsonWriter and JsonSerializer.

Json.NET was first released in June 2006. Since then Json.NET has been downloaded thousands of times by developers and is used in a number of major projects open source projects such as MonoRail, Castle Project's MVC web framework, and Mono, an open source implementation of the .NET framework.
轉自：http://james.newtonking.com/projects/json-net.aspx