I usually use photoshop’s save for web and devices, but it has no options for compressing pngs and I also wonder if there are superior compression algorithms available?
If the target file type is PNG, then you have a few options. First, open the PNG in Fireworks then save optimized. FW compreses .png files better than PS. Then use PNGOUT(freeware) to compress further. Typical result is about 30% less than what Photoshop would produce.