optiPNG

[Purplecat]

Optimizes PNG by making them smaller, yet without losing picture quality. Having lots of PNG pictures, I obviously fell in love with this program. It's very safe, and I hadn't heard anyone losing pictures with this tool (and there's a parameter to keep the orginal pictures if you're paranoid).

http://optipng.sourceforge.net/

There's this one frontend for it, with obviously Engrish on it's webpage. I would advise against using it, as the commandline utility is simple enough (unless you for some reason lost, is very illiterate or broke your keyboard).

[Purplecat]

For anyone nerdy enough, comparing backup files with the optimized pictures. It didn't backup a few files due to my own mistakes, but intresting that I shaved off 25% in average.

[Fernin]

A nifty tool, most of my picture collection is PNG. I'll have to give this thingie a looktry.