Pngcrush is an optimizer for PNG (Portable Network Graphics) files.
It can be run from the commandline.  Its main purpose is to reduce the
size of the PNG IDAT datastream by trying various compression levels
an PNG filter methods.  It also can be used to remove unwanted
ancillary chunks, or to add certain chunks including gAMA, tRNS, and
textual chunks.

Pngcrush is open source and may be used by anyone without fee. 

The author of pngcrush is Glenn Randers-Pehrson, randeg@alum.rpi.edu