http://psydk.org/PngOptimizer.php
Can also optimize and convert bmp,gif,tga files into png files.Been using it for a few months and have been very satisfied with it’s result.
I tend to stick to the gif file format for posts to the forum as a) the quality is fine for that purpose and b) generally the gif file format produces a much smaller file size, much better for us poor saps on dial-up.