Maybe you’ve heard of things like Pngcrush before which supposedly does an amazing job at optimizing png files, but then when you go check it out it turns out it’s this nerdy command line tool with crappy documentation and it just doesn’t look like it’s worth the hassle. Enter ImageOptim, which is a Mac app with a super simple user interface for using Pngcrush as well as a bunch of other similar awesome compression tools. Literally just drag and drop the images into the window and it does its thing.
Why do you need this? If you are “Save for Web”ing from Photoshop, that just isn’t as good as you can do. I’ve been using it and saving 8% on average of lots of different images. That’s a significant speed boost on the web.