[ajslater]

I've had the best luck using opitpng and then pngout.

https://github.com/ajslater/picopt ...will do this automatically for you.

Pictopt: A multi-format, recursive, multiprocessor aware, command line image optimizer utility that uses external tools to do the optimizing.

For an all in one GUI for Mac approach to this, try ImageOptim

[imaginenore]

PNGOut is definitely better than OptiPNG most of the time:

http://www.olegkikin.com/png_optimizers/

[jibsen]

zopflipng (with -m switch) is close to or better than the best on those five images:

    1_zopflipng.png  8.466
    2_zopflipng.png 18.739
    3_zopflipng.png  8.012
    4_zopflipng.png 91.879
    5_zopflipng.png  1.117