ImageOptim - Faster web pages and apps.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
pngquant - command-line utility and library for lossy compression of PNG images
ImageMagick - ImageMagick is a software suite to create, edit, and compose bitmap images.
PNGGauntlet - -Combines PNGOUT, OptiPNG, and DeflOpt to create the smallest PNGs