DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
ImageOptim - Faster web pages and apps.
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
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.