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
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.
OptiMole - Optimole makes your images weigh even 80% less while looking just as good!
Minuimus - Transparent automatic compression of a wide array of file types, using well established open source utilities.