ImageOptim - Faster web pages and apps.
ConvertIcon! - Converticon is a simple icon utility.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
ConvertICO.org - Create icons from png, jpg or bmp images easily.
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.
Ordinatore - Those who want to tidy up their Macs will surely love this application.