There are so many different json libraries in .NET, which provide for some sort of serialization. Look at the bottom of http://json.org/ to find a list of JSON libraries for C#. Whichever one is best depends on your needs. I've used json.net from Newtonsoft, although personally I have found the serializer to be very slow. If you need speed Servicestack claims to have the fastest one (here). The site also shows some performance comparisons, although I have not tried it.
Do some Googling, or simply try them out, write some tests, or just pick the framework one and introduce no extra dependencies and be done with it.