Guetzli is an advanced JPEG encoder, with a complete new psycho-optical model, introduced by Google, claims better visual result with smaller file size. And it’s 100% compatible with the JPEG format.
Google provide a command line tool for the Guetzli encoder, although with this Windows front-end you can easily put files to an encode queue, select the output folder and the encoding quality.
This program is open source software, distributed under the terms of the GNU GPL 3.
For more info, updates and downloads see at the Project’s Home Page.