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 Macromedia Adobe Fireworks by employing latest pngquant and pngnq-s9 and alpha-channel-aware posterizer.

Drew Albinson
Source: ImageAlpha