TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
E-mage - From developers to developers: a cross-platform tool for losslessly image compression.
TinyJPG - Compress JPEG images with perfect quality and file size
JPEGmini - JPEGmini - The Photo Optimization Tool Trusted by Tens of Thousands Image Perfectionists
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
ImageOptim - Faster web pages and apps.