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.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
ImageAlpha - ImageAlpha is a GUI for pngnq and pngquant.
Compressor.io - Optimize and compress your images online. Compressor is a lossy and lossless photo compression tool.