Vallen jpegger - Vallen JPegger is the international, handy, and full featured image viewer, which shouldn't be...
ImageOptim - Faster web pages and apps.
Cloudimage - Cloudimage.io is the easiest way to resize, store, and deliver your images to your customers through a rocket fast CDN.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
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.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!