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.
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
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...