Although pngcrush does reduce image sizes, there are still rooms for improvement. PNGCrush should use more compressive implementation of deflate algorithm, making it comparable to PNGout.
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There is one called AdvanceCOMP, which is hosted in SourceForge as well, but it is licensed in GPL. It is not likely to be added unless PNGCrush's licence is changed.
I've been experimenting with zopfli (see http://pmt.sf.net/pngzop) which almost always seems to achieve a few percent better compression than pngcrush.