See the difference between these two icons?  Me neither, but the one on the right has about an 8% smaller file size. Portable Network Graphics (PNG) is quickly becoming the prefered format for web graphics.  It supports alpha transparency for nice blendy graphics (now in most browsers), and has amazingly small file sizes.  But those small file sizes can be made even smaller thanks to some custom compression tricks and pulling out unneeded data.  PNGOUT is a great little command-line utility for doing just that. 

Since it's a command line app, it can be integrated easily into your build process to automatically compress any PNG files in your images folder, so that no matter where your PNGs come from, you can ensure they take up as little bandwidth as possible.