XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...
ImageOptim - Faster web pages and apps.
pngquant - command-line utility and library for lossy compression of PNG images
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
Phatch - Phatch = Photo & Batch!