PNGs are pretty standard images on the web these days, but Photoshop tends to generate unnecessarily large files. And even though most people have high-speed access, any savings in image size is a good thing and will help speed page load times.
Enter PNG Thing for Mac. Running your Photoshop-generated PNGs through PNG Thing reduces their size: just drag your images onto PNG Thing and strips out unnecessary junk, making your PNGs smaller. (It does a better job on large files and doesn’t make much of a dent with smaller PNGs.)
Dave Shea says Fireworks does an even better job of reducing PNGs. But if you don’t have Fireworks handy, PNG Thing is a nice (free) way to go.