Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
Squash 2 for Mac - The easiest way to compress and optimize images for the web
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