What are some alternatives?
TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
JPEGmini - JPEGmini - The Photo Optimization Tool Trusted by Tens of Thousands Image Perfectionists
PNGGauntlet - -Combines PNGOUT, OptiPNG, and DeflOpt to create the smallest PNGs
WP Smush - Reduce image file sizes, improve performance and boost your SEO using the free WPMU DEV WordPress...
pngquant - command-line utility and library for lossy compression of PNG images
Optimus.io - Lossless image optimization