The main problem with PNG files is that you get a PNG image right after saving in a program like Photoshop or Fireworks, and these PNGs are pretty big files. Thanks to PNGGauntlet, you can fix exactly this problem.
The program is so easy that you just have to drag the image over the program's window and choose what file you want to save the image to. In just a few seconds you'll have a compressed image without a loss in quality.
PNGGauntlet is rather useful if you want to upload a high quality image to the internet without using up so much space.