ImageOptim - Faster web pages and apps.
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
jStrip - jStrip is a free tool to reduce the file size of JPEG images without sacrificing image quality.
Ralpha Image Resizer - High-speed image batch conversion tool
ImageAlpha - ImageAlpha is a GUI for pngnq and pngquant.
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.