TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
ImageOptim - Faster web pages and apps.
Optimus.io - Lossless image optimization
OptiPNG - OptiPNG is a command-line PNG optimizer that recompresses image files to a smaller size, without...
OptiMole - Optimole makes your images weigh even 80% less while looking just as good!
TruePNG - Command-line PNG optimizer.