PNGGauntlet is a .NET program that uses Ken Silverman's PNGOut to optimize PNG files. Many PNG files output by popular graphics software packages like Photoshop or even Fireworks are not as small as they could be - PNGGauntlet squeezes the last bit of size out of them.
Furthermore, no information is lost during this process - the only thing that changes is the file size. Just select the files you want to optimize (or drag them onto the window), select where you want them to go, and click the button.
PPNGauntlet can also be used to convert JPG, GIF, TGA, PCX, and BMP files to PNG. PNGGauntlet also supports a brute-force option that will try different PNGOut settings in order to create the smallest file.