Hey I found a good little app today. PNGGauntlet.
Ok, so what is it? Well it squashes PNG images and makes them smaller. Seems to work very well with no loss of quality. It will handle both png-8 and png-24 images, and you can even feed gifs or whatever into it and it will spit them out as png's.
I've been doing a bit of testing and depending on the image I've seen file size reductions of between 20% and 60% compared to a png-24 exported straight from Photoshop and around 10% to 20% on png-8's.
It comes with a simple and easy to use GUI, which is handier than some of the other command line tools around, and it seems to give better compression than the others as well. Highly recommended if you're looking to get your images loading faster and using less bandwidth.