[asuth]

this can be greatly improved (losslessly) using http://punypng.com / http://www.smushit.com/ysmush.it/ / downloadable PNG recompressing software. I tried it on several files on this site and some decreased in size by more than half.

[malkia]

is that better than pngcrush? http://pmt.sourceforge.net/pngcrush/

[JonnieCache]

I've always had good results with pngout