ImageAlpha - ImageAlpha is a GUI for pngnq and pngquant.
jStrip - jStrip is a free tool to reduce the file size of JPEG images without sacrificing image quality.
ImageOptim - Faster web pages and apps.
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
Optimage - Advanced image optimization tool
Tinybeest - A Mac app and Chrome extension that optimizes/compresses images and PDFs.