ImageAlpha greatly reduces file sizes of 24-bit PNG files (including alpha transparency) by applying lossy compression and conversion to a more efficient PNG8+alpha format. Such images are compatible with iOS, all browsers, and even degrade well in IE6.
ImageAlpha can achieve better quality than similar function in Adobe Fireworks by employing latest pngquant and pngnq-s9 and alpha-channel-aware posterizer.
How to use it
Drag true-color PNG image into main part of the window. Small images should be converted instantly. If image is large, there might be progress spinner visible for few seconds.
Available from command line. ImageAlpha is based on pngquant. You'll find compiled pngquant executable in ImageAlpha.app/Contents/Resources directory.
ImageAlpha is free, open-source software under terms of the GPL v2. Your contributions are welcome! Please check out/fork the source code (Python) and improve it! Feel free to contact me for assistance.