pngquant is a command-line utility to quantize and dither 32-bit RGBA
PNGs down to 8-bit (or smaller) RGBA-palette PNGs, usually with a
significant reduction in file size (40-70% smaller than 24-bit PNGs).

This unique type of PNG supports full alpha transparency and is
compatible with all modern web browsers, and has better fallback in IE6
than 24-bit PNGs.

Features:
 - High-quality palette generation using modernized Median Cut
   algorithm.
 - Unique dithering algorithm that adds less noise to images than the
   standard Floyd-Steinberg.
 - Easy to integrate with shell scripts, GUIs and server-side software.
 - Fast mode for processing large numbers of images.

WWW: http://pngquant.org/