Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
SVGO - Tool for optimizing SVG files
ImageOptim - Faster web pages and apps.
SVGminify.com - Removes superfluous information reducing the size of SVG files
E-mage - From developers to developers: a cross-platform tool for losslessly image compression.