punypng is a great service for those who work with the .png image format. If you work on web sites, presentations, or other projects that involve image files, this is really worth adding to your toolkit.
What it does is fairly straightforward: you upload your .png image file to their site, their algorithm examines the file and determines what can be done to make the file smaller without sacrificing image quality, and offers you the file as a download.
I tried it, and got as much as a 48% reduction in file size, without any visible loss of quality. Even if all you get is a 10% reduction in file size, it means that your web sites, presentations, etc. will load and run faster. For a busy web site, like Google or WorldCat, every reduction in the data needed to be sent to each user can add up to large savings in time, energy, and ultimately money.
found via SitePoint