PNGOptim

Designed as a graphical interface for PNGOut, PNGOptim is an easy-to-use Java-based tool designed to optimize PNG images in bulk, in order to reduce the size they occupy on the disk.
It automatically does this by taking into account predefined settings, so it can be used even by those inexperienced with such software.

No setup required, besides Java

The app's not wrapped in a setup kit, so you can copy the downloaded file to a custom location on the HDD and just double-click it to launch the program. Just keep in mind that you must have Java installed, since it was developed using this platform.
On the other hand, it doesn't change the Windows registry configuration, need other DLLs to run, or create extra files on the disk without your permission.

Automatically optimize PNG photos

As far as the interface is concerned, PNGOptim opts for a normal window with a plain and simple structure, where all you have to do is add PNG images to the task list via drag-and-drop support. It's also possible to drag directories to take into account all containing PNG files.
The optimization process is immediately carried out, after which you can check out the name, status and size difference (percent) of each file. Apart from the fact that you can recursively process the directories, there are no other noteworthy options available.
Worth noting is that the utility processes the original files, so you may be interested in performing backups before resorting to this application, in case you're not satisfied with the post-job results.

Evaluation and conclusion

Optimization jobs were carried out swiftly in our tests, during which PNGOptim remained light on system resources usage. It didn't hang, crash or indicate errors. Although it's not that feature-rich, PNGOptim provides you with a speedy solution for PNG optimization.