Handy reference comparing the PNG compression capabilities of three graphics apps (bottom of page). Also an overview of commonly used tools for PNG compression.
There’s still an amazing amount of FUD doing the rounds about PNG, which is unfortunate. I think the main issue now is that many artists get burned by PNG because they don’t get the filesize savings they’ve been promised compared to GIF. The savings are there, it’s just that there are so many “correct” ways to write a PNG that you can get a range of file sizes from far too large to ultra-lean. I once again recommend PNGout as a simple tool to reduce PNG file-sizes at the very last stage of image creation.