ImageOptim is a free app that makes images take up less disk space and load faster, without sacrificing quality. It optimizes compression parameters, removes junk metadata and unnecessary color profiles.

ImageOptim seamlessly integrates the best optimization tools: PNGOUT, Zopfli, Pngcrush, AdvPNG, extended OptiPNG, JpegOptim, jpegrescan, jpegtran, and Gifsicle.

It's excellent for publishing images on the Web (easily shrinks images “Saved for Web” in Photoshop) and also useful for making Mac and iPhone/iPad applications smaller (if you configure Xcode).