Implementations

PHP: ARC2
Java: Jena
C: Redland librdf added in [version 1.4.17] (<a href="http://lists.librdf.org/pipermail/redland-announce/2008q1/000078.html" rel="nofollow">http://lists.librdf.org/pipermail/redland-announce/2008q1/000078.html ) of the Raptor parser component.
Ruby: RDF.rb includes an RDF::JSON plugin
Python: RDFLib includes an add-on RDF/JSON parser/serialiser module

A listing of known implementations of the RDF/JSON spec