TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
Shrink O'Matic - Shrink O'Matic is an AIR application to easily (batch) resize (shrink) images.
ImageOptim - Faster web pages and apps.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
TinyJPG - Compress JPEG images with perfect quality and file size
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.