This is a package that allows conversion to and from data in
Javascript object notation (JSON) format. This allows R objects to
be inserted into Javascript/ECMAScript/ActionScript code and allows
R programmers to read and convert JSON content to R objects. This
is an alternative to rjson package. That version was too slow for
converting large R objects to JSON and is not extensible, but a
very useful prototype. It is fast for parsing. This package uses
methods, vectorized operations and C code and callbacks to R functions
for deserializing JSON objects to R. Verison 0.4 of this package
uses a new native parser, implements the transformation code in C
and allocates memory efficiently (rather than concatenating because
of event driven parsing). The result is a significantly faster
parsing of large JSON documents.

WWW: http://cran.r-project.org/web/packages/RJSONIO/