SuperPNG for your downloading pleasure. The big new feature is the Quantize checkbox, which will convert your 32-bit RGBA image into an 8-bit indexed color image with transparency. And I'm not talking about GIF-style 1-bit transparency here, but full 8-bit transparency values in the PNG color palette.
That 75% reduction in bits can really crush PNGs even more than was previously possible. Lower the quality slider to use fewer colors, which means even fewer bits and an even smaller file. This feature is powered by Kornel Lesiński's pngquant library, and Kornel has agreed to join me as SuperPNG's co-author.
Another new feature in this version is the Clean Transparent checkbox. It turns out that your transparent pixels may have unseen colors in them that are making PNGs harder to compress, but this feature will wipe them out for you. See the manual for more details.
SuperPNG has always been free, but now it's open source as well. Check out the GitHub repository and see exactly what we're doing.