ImageOptim - Faster web pages and apps.
FILEminimizer Pictures - FILEminimizer Pictures compresses JPEG photos, TIFF, BMP and PNG images and pictures by up to 98%.
jStrip - jStrip is a free tool to reduce the file size of JPEG images without sacrificing image quality.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
ImageAlpha - ImageAlpha is a GUI for pngnq and pngquant.
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.