pngcrush reads in a PNG image, and writes it out again, with the
optimum filter_type and zlib_level.  It uses brute force (trying
filter_type none, and libpng adaptive filtering, with compression
levels 3 and 9).  It does the most time-consuming method last in case
it turns out to be the best.  Optionally, it can remove unwanted
chunks or add gAMA and sRGB chunks.

pngcrush does not require external X/png libraries.

WWW: http://pmt.sourceforge.net/pngcrush/