PNGOUT is a free optimizer for PNG images and is written by Ken Silverman. The compression is lossless, meaning that the resulting image will have exactly the same appearance as the source image. This program can often get higher compression than other optimizers by 5-10% Wikipedia
A few weeks back Scott Hanselman posted a blog article about PNGOUT. I finally got around to trying it and Wow does this thing work. It can sometime shrink PNG’s by 50%. Scott published a registry file to add a right-click shortcut menu in Windows Explorer. I’ve found that PNGOUT also converts JPG’s and BMP’s to PNG’s and shrinks them with even more stunning results. I’ve created a registry file that registers these extensions as well as PNG.