ImageOptim - Faster web pages and apps.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
FileOptimizer - FileOptimizer is a lossless file size optimizer supporting AIR, APK, APNG APPX, BMP, CBZ, DOCX...
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
pngquant - command-line utility and library for lossy compression of PNG images
Squoosh - Compress and compare images with different codecs, right in your browser