Jsonspirit is a project mainly written in C++ and C, it's free.
JSON parser and generator using boost/spirit
JsonSpirit tries to implement a very script (read: ruby, python, perl) like interface to json structured data without loosing type safety.
It is build around a single type (json) that provides functions to parse (e.g. from_string) or serialize (e.g. to_string) as well as querying the type and converting the type if neccessary.
No external syntax file or parser generator is used, the parser is written in C++ useing the awesome spirit library.