Pngrewrite is little utility that reduces the unnecessarily large palettes that too many programs write into PNG files. It also optimizes transparency settings, and reduces the bits-per-pixel if possible. Handy for post-processing images before putting them on a web site.
Download:
Read the comments at the beginning of the source code (pngrewrite.c) for more details.
See also
PNGGauntlet, a friendly interface for pngrewrite and other utilities.
pngcrush by Glenn Randers-Pehrson. Tries to optimize the compression of the image. If you use pngcrush, make sure to run it after pngrewrite, not before.
OptiPNG by Cosmin Truta. Similar to pngcrush. Newer versions also include much of pngrewrite's functionality.
PNGOUT by Ken Silverman.