Because PNGGauntlet uses PNGOUT, so image optimization is completely lossless. PNGauntlet can be used to convert JPG, GIF, TGA, PCX, and BMP files to PNG, and reduce size of even existing PNG files. PNGGauntlet filters out the unnecessary bytes to reduce image size.
PNGGauntlet has a nice and easy to use interface. Just add the files that you want to optimize, select your basic options, and start Optimization. PNGGauntlet is able to reduce size of multiple files together.
PNGGauntlet is completely free, and is a nice UI extension to PNGOUT. Download PNGGauntlet using links at the end.