TinyPNG is an awesome tool/website. All you do is drop a .png file that you need to compress onto the page and TinyPNG will compress it up to 76%, alpha transparency preserved.
So… how does it work?
“TinyPNG uses smart lossy compression techniques to reduce the file size of your PNG files. By selectively decreasing the number of colours in the image, fewer bytes are required to store the data. The effect is nearly invisible but it makes a very large difference in file size!”