json.org has a nice javascript library
simple usage:
JSON.parse('[{"some":"json"}]');
JSON.stringify([{some:'json'}]);
Edit: As pointed out in comments, this uses eval if you look through its source (although it looks to be sanitized first)
to avoid it completely, look at json_parse or json-sans-eval
json2.js is insecure, json_parse.js is slow, json-sans-eval.js is non-validating