OptiPNG is a PNG optimizer that recompresses image files to a smaller size without losing any information. …
It is found in many distributions' package managers, so installation is easy. On a Mandriva box, that would be urpmi optipng as root.
Basic usage is as simple as optipng filename.png. See the manpage for advanced examples.
I like to add optipng as an editing option to gqview, my preferred thumbnail viewer. In gqview, open File – Preferences – Editors. In a blank menu entry, add (for example) "Optimize PNG" as a menu name and "optipng -fix -preserve -q %f" (or your preferred options) as the command line. Once configured, use is simple: right click on the image to be optimized (either the thumbnail or the large image) and select "Edit – in Optimize PNG".
Of the alternatives I tried, OptiPNG has the best documentation and gives me the smallest output sizes. But it it's not what you're looking for, there's also pngcrush and pngrewrite.
PNG is a fine format but it isn't a one-size-fits-all solution. Consider other formats as well.