There are various PNG optimizer apps for Mac OS X (check this WRD article for them) but only a few for Windows.

PNGGauntlet is a free Windows application for squeezing each and every byte in PNG files.

With a very nice drag 'n' drop enabled user interface, the application can optimize any given number of files pretty quickly.

PNGGauntlet

It combines the algorithms of PNGOUT, OptiPNG, and DeflOpt for creating the smallest file and generates a lossless output.

The image to be minified doesn't need to be in PNG format, other popular formats can be used as well for converting them to PNG.

There are also several settings for anyone looking to fine tune the compression process.

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