TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
ImageOptim - Faster web pages and apps.
XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...
TinyJPG - Compress JPEG images with perfect quality and file size
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
Optimizilla - Optimizilla is an online image optimizer uses a smart combination of best optimization and lossy compression algorithms in order to shrink JPEG and PNG pictures to the minimum possible size while keeping the required level of quality.