Subscribe to Comments Reloaded allows commenters to sign up for e-mail notifications of subsequent replies.
Download at WordPress.org: http://wordpress.org/plugins/subscribe-to-comments-reloaded/
If you would like to contribute to the development I strongly recommend you to pull your code request to the development branch instead. The master branch is only a copy of the WordPress version and will only be updated on releases.
StCR development version now uses Gulp and Composer.