After dragging my JSON implementation through various projects, and having several incompatible versions exist at the same time, I got permission to open-source it from the company that originally paid me to write it, so it has a home now: ST-JSON.
There already exists a comparable library called CL-JSON. I
originally wrote a new one because the way CL-JSON uses nil to
encode all of boolean false, the empty list, and the empty object was
causing headaches, and later I added some other extensions.