ImageOptim - Faster web pages and apps.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
jStrip - jStrip is a free tool to reduce the file size of JPEG images without sacrificing image quality.
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.
ImageAlpha - ImageAlpha is a GUI for pngnq and pngquant.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.