Image Optimizer is a tool which can apply lossless compression to PNG files, reducing their size by up to 40% without any reduction in image quality.
The program's interface is poor, but its operation is simple enough. Drag and drop your files onto the work area, select File > Optimize All Files & Save to a Different Folder, and wait. When the compression process is over, a folder will open with your slimmed-down files and you can view the results.
We tried Image Optimizer on our test images, and it did very well. Our real-life web graphics (PNGs taken from a variety of websites) were reduced by an impressive 39%. Compressing high resolution photos is much more difficult, but even here the program was able to achieve something, shaving 4.5% off the size of our pictures.
There are also some down sides here. This is a trial version which expires after you've run it a certain number of times (around ten, we think, although this doesn't seem to be mentioned anywhere). And there are no details on pricing, either. So while the program does work well, use it with care, and get the very maximum value from its first few runs.
Image Optimizer has a poor interface, and has no indication of how much it costs. The program does offer good lossless PNG compression rates, though - without adding watermarks - so if that appeals then we'd give it a look.