Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...
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.
TinyJPG - Compress JPEG images with perfect quality and file size