[You can download and read all about PNGPress at it’s homepage][link]. It’s essentially a Applescript front-end for [OptiPNG][opti], the command line application.
*Update:* The download link for PNGPress is now fixed. Whoops.
[link]: http://extrafuture.com/code/pngpress/ “PNGPress”
[opti]: http://optipng.sourceforge.net/ “OptiPNG”