PNGout
pngout is a command line tool developed by Ken Silverman for optimising PNG files losslessly. I've been using it for a while and in fact, I haven't found anything that performs better than it. Even with the default options, it optimised some of my PNG's by up to 50%.
For those who don't know, Ken Silverman is the programmer that created the game-engine for Duke Nukem 3D. Silverman received lots of praise from John Carmack - which says a lot because Carmack is co-founder of ID Software and lead developer for games like Doom and Quake (he also wrote what is likely the fastest inverse-square-root function, but that's a topic for another day).
For those not comfortable with command-line, even after reading this tutorial - there's an IrfanView DLL available.
To run with default options:
pngout.exe png-file-original.png png-file-output.png
pngout is available from http://www.advsys.net/ken/utils.htm
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