ImageOptim - Faster web pages and apps.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
TinyJPG - Compress JPEG images with perfect quality and file size
FileOptimizer - FileOptimizer is a lossless file size optimizer supporting AIR, APK, APNG APPX, BMP, CBZ, DOCX...