RMS wrote, several months ago, about my json.el:

> If a lot of people start wanting this facility, then it would be good
> to include it in Emacs. Please let me know if that happens.

I know of several libraries/tools which use json.el, including
augment.el, a facebook api client, rcov.el, and at least one twitter api
client. Additionally, I know of at least one library, twit.el, whose
author avoided using json.el solely because it isn't included in Emacs.
From twit.el's commentary:

> This uses Twitter's XML-based api, not the JSON one because i would
> like to avoid making the user install third-party libraries to use it.

I'm not sure if this counts as "a lot," but I would be quite happy to
include json.el with Emacs if you'd like it. I believe I have the
relevant papers on file.

I've attached the latest version for your perusal.