Third-party-json-adapters is a project mainly written in Objective-C, it's free.
Third Party JSON Adapters for NSJSONSerialization in iOS 5
As most of you know, Apple made their JSON parsing framework as a non-private API in iOS 5.
This class will map the categories on SBJSON to NSJSONSerialization.
Volunteers wanted for writing similar adapters for other frameworks that are slower than NSJSONSerialization