ImageOptim - Faster web pages and apps.
Oxipng - Oxipng is a multithreaded lossless PNG compression optimizer. It can be used via a command-line interface or as a library in other Rust programs.
Minuimus - Transparent automatic compression of a wide array of file types, using well established open source utilities.
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
E-mage - From developers to developers: a cross-platform tool for losslessly image compression.