If you want to get the free version and not bothered to write some code every time you want to compress your png images, then go download PNGOUT and learn how to use it. Additionally, if you like it, read here also to find out how to integrate PNGOUT to Windows explorer context menu.
On the other side, PNGOUTWin is not free, but it surely is the best option if you want to do batch png compression in as easy as drag and drop the images to the GUI interface. Add png images as many as you need, get them shrunk, and you will probably get surprised with the result.

Beside compressing your existing png images, it can also be used to convert other image formats like bmp, tiff, jpeg, gif to png.
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