NextGEN Gallery is a free and open-source image management plugin for the WordPress content management system. It has been downloaded 15 million times (as of July 2016), making it the fifth most popular plugin for WordPress.
Licensing
Like WordPress, NextGEN is released and governed by the GNU General Public License. NextGEN's source code is publicly available from its Bitbucket repository.
References
NextGEN Gallery Wikipedia(Text) CC BY-SA