PngOptimizer makes smaller PNG than PngOut
Many designers/developers have discovered and used PNGOUT for awhile now to make smaller (sometimes significantly smaller) PNG files with better compression. I recently discovered another GPL program, PngOptimizer that makes even smaller files in many cases, especially for PNG8 files (256 colors with transparency, similar to GIF but better in some cases and IE supports it).
http://psydk.org/PngOptimizer.php
As a bonus it already has a drag-n-drop menu for multiple files where PNGOUT requires some extra 3rd party programs and the source is available. It’s perfectly safe to run files through PngOptimizer and then PNGOUT to find which one will make it the smallest as all the compression remains “loss-less” and PNGOUT won’t replace files that become larger, only smaller.
Comparison: FamFamFam.com v1.3 icon set
original: 662,196 bytes
pngout: 660,407 bytes (at “extreme” setting)
pngopt: 605,545 bytes
That’s over 50k better !
_ck_ says... 
nice info & TQ
March 8, 2009 at 3:47 am
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I think I better clarify this post.
PngOptimizer is better than PngOut on SMALL files (like icons).
Once you start getting into images that are dozens or hundreds of K, the output size is about the same size between the two programs.
March 27, 2009 at 11:35 pm
I’ve found that if you make PNG8 using ImageAlpha/pnguant app (http://pngmini.com) then they compress even better (well, FamFam is not a great example of this because files are tiny to begin with, but it makes big difference on large files).
I must say, pngopt does wonders. With ImageOptim I got it down to 620KB.
December 17, 2011 at 11:16 am
99% file size after PNG optimizer… kinda useless.
February 4, 2014 at 6:44 pm