PNGOUT (http://advsys.net/ken/utils.htm#pngout http://advsys.net/ken/utils.htm#pngout) and PNGCRUSH (http://pmt.sourceforge.net/pngcrush/ http://pmt.sourceforge

PNGOUT (http://advsys.net/ken/utils.htm#pngout http://advsys.net/ken/utils.htm#pngout) and PNGCRUSH (http://pmt.sourceforge.net/pngcrush/ http://pmt.sourceforge.net/pngcrush/) are similar utilities that can compress PNG files even further. The idea is that you do all your invasive stuff (like reduce colors etc.) in Save for Web in Photoshop (or similar) and then let one of the two programs do its job, which will visually not alter the image.