ImageOptim - Faster web pages and apps.
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
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.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.