Here is a handy little program: pngrewrite. I came across it in the FreeBSD ports collection. It's a PNG optimiser, which is particularly good for images with few colours, as it converts them to indexed with a minimal palette. All the Kye screenshots (the graphics for which date from 16-colour VGA days) are 40% smaller after going through pngrewrite.

It looks like it is freely-usable source-code-available only (not open source); none of the open source alternatives seem to be half as good. Only with pngrewrite can I get the tiles for Kye to be smaller as PNG than as GIF.