PNGGauntlet

Program: PNGGauntlet
License: Freeware ("The software license for PNGGauntlet permits unlimited distribution as long as nobody charges money for it, modifies it, or packages it with other software without [the developer's] permission. It also frees [the developer] from any liability from your use of the software. A copy of the actual license is included in the installation and is viewable from the "About" dialog."). The developer's e-mail adress is ben [at] benhollis [dot] net, ask him and he might give you permission.
Description: PNGGauntlet is the world's most effective lossless PNG optimizer, which will compress PNG files so that the filesize decreases while the image quality is identical.
Website: https://pnggauntlet.com/
Other: Requires .NET Framework 4.0