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There's already a "mozpng", it's called Zopfli. There's also AdvPNG which compresses PNGs with 7-zip's deflate implementation.

And if you want much much smaller PNGs, then try http://pngquant.org or http://pngmini.com/lossypng.html

Or try ImageOptim, which bundles these and other tools with a nice GUI: http://imageoptim.com/

ImageOptim also bundles PNGOUT, which I find to have exceptional compression when compared to the others (but it is a bit slower)

See my comment about zopfli [1] for improved png compression algorithm.

Mozilla already uses patched libpng (with APNG support).