The JSON-Fortran source code and related files and documentation are distributed under a permissive free software license (BSD-style). See the LICENSE file for more details.
The current stable release is 4.2.0 and can be downloaded on GitHub or installed with Homebrew on Mac OSX. The documentation for the current version, 4.2.0, can be found here1, and a list of changes from the previous version are here.
A list of all past releases, links to their documentation, and the chage log can be found on the releases page.