RawTherapee is a cross-platform raw image processing program, released under the GNU General Public License Version 3.

PNGGauntlet is a PNG compression app for Windows.

FCorp Imaging is a software suite of image related tools.

Radical Image Optimization Tool (RIOT) is a free image optimizer that will let you to visually adjust compression parameters.

OptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information.