What are some alternatives?
Lepton - Lepton image compression: saving 22% losslessly from images at 15MB/s
OptiPNG - OptiPNG is a command-line PNG optimizer that recompresses image files to a smaller size, without...
File Converter - Convert & compress everything in 2 clicks!
pngCrushrrr - Drag your PNGs onto PNGCrushrrr and get super-optimized files ready to be emailed or posted on the...
ImageOptim - Faster web pages and apps.
CodeKit - CodeKit allows you to optimize the performance of your website by automatically and efficiently compiling a variety of popular languages.