TruePNG - Command-line PNG optimizer.
E-mage - From developers to developers: a cross-platform tool for losslessly image compression.
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!
JPEGmini - JPEGmini - The Photo Optimization Tool Trusted by Tens of Thousands Image Perfectionists
pngcrush - Pngcrush is an optimizer for PNG (Portable Network Graphics) files.