ImageOptim by Kornel Lesiński and contributors is a GUI for 3rd party utilities:

• OptiPNG by Cosmin Truta,
• PNGCrush by Glenn Randers-Pehrson,
• Zopfli by Jyrki Alakuijala, Lode Vandevenne,
• AdvPNG by Andrea Mazzoleni, Filipe Estima,
• Jpegoptim by Timo Kokkonen,
• Gifsicle by Eddie Kohler,
• and PNGOUT by Ken Silverman.

ImageOptim can be redistributed and modified under GNU General Public License version 2 or later. Bundled PNGOUT is not covered by the GPL and is included with permission of Ardfry Imaging, LLC.