ST-JSON ('ST' because it originated at Streamtech) is a Common Lisp library for encoding and decoding JSON values (as specified on json.org).
This library does mostly the same thing as CL-JSON, but is simpler and more precise about types (distinguishing boolean false, the empty array, and the empty object).
The library is available under a zlib-style license.
The easiest way to install is with QuickLisp:
(ql:quickload :st-json)
It can also be installed with asdf-install, or you can download the current release manually. The git repository can be checked out with:
git clone http://github.com/marijnh/ST-JSON
See the webpage for documentation.