If you want something that does both (the gamma removal of GammaSlamma combined with the crushing of Ping), you can use the widget PNGpong. It's not a standalone unfortunately, but it still works really well so I have since switched (it uses both the pngcrush and the optipng "engines" together).
@Pilot_Pirx, PNGpong also allows the option of overwriting the existing file instead, too. :-)

Removing the gamma info not only make PNGs display consistiantly in all browsers but also make the RGB-values in the picture display consistantly with the RGB-values in CSS. This is very helpful - for example - to make a PNG fit into a CSS-defined background-color.
There is only one feature in GammaSlamma that Im missing. I'd like to have the option to automatically replace the original file by the slammed one.

@cbserra: the only thing that can fix that is the IE developers :\

@cbserra: Nope! :-/

Silly me for believing that the gamma information was supposed to enable a consistent display... oh wait now, it's in the spec: "gamma correction for cross-platform "brightness" control".

If you read the article the author's page links to, it too says the problem is browser support, not PNG itself. So make of it what you will. Personally I still get cheesed by JPG profiles (akin to this PNG problem). PNGs give me the least trouble, and I use them as much as GIF and TIFF for different reasons.

@cbserra, that's a different kind of browser issue that really only affects IE. I'd say no.

by removing the gamma info, does this allow PNGs with transparency to display in IE6 without applying a hack to the HTML?