ImBatch - ImBatch is a batch image processor with a nice graphical user interface.

ImageOptim - Faster web pages and apps.

XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...

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.

DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.

Minuimus - Transparent automatic compression of a wide array of file types, using well established open source utilities.