Batch optimize in various compression levels PNG files easily, with Punig GUI for OptiPNG
Punig is a free and portable application, a Graphical User Interface for OptiPNG, having OptiPNG embedded into its own executable.
You can drag files to Punig (unsupported files are ignored) or you can click the Add Files button.
There is a “Mass” button which searches a directory for all supported files. You can also set compression levels.
Since version 11 Punig supports multi-core processors.