ImageOptim - Faster web pages and apps.
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
Shrink O'Matic - Shrink O'Matic is an AIR application to easily (batch) resize (shrink) images.
Squash 2 for Mac - The easiest way to compress and optimize images for the web
FILEminimizer Pictures - FILEminimizer Pictures compresses JPEG photos, TIFF, BMP and PNG images and pictures by up to 98%.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.