PNGPress

PNGPress squeezes every last kilobyte out of PNG files without degrading quality.
To compress images (PNGs only) drag any number of them to the PNGPress icon in the dock.

PNGPress will work its mojo and give you a report on the before and after sizes of your images.
Some applications compress PNGs using a method similar to PNGPress, which will result in PNGPress being unable to compress them further, or only being able to compress the image by a small percentage.