PngOptimizer rewrites PNG files in an efficient way, stripping out any redundant information, hopefully reducing their size without any loss of image quality.
The program really couldn't be much easier to use. Drag and drop your target images onto PngOptimizer and it compresses each one in turn. The new files are saved using the same name and location as the source, while the original files are renamed as a backup (file.png becomes _file.png).
As a bonus, PngOptimizer also supports the import of BMP, TGA and GIF files, also converting these to PNG in much the same way.
Does it work? We gave PNGOptimizer a really tough test, pointing the program at 25 high resolution photos - an average of 8MB each - in PNG format. These took a little while to process, unsurprisingly, but the results weren't bad at all, with our files losing an average of 4% each.
While that may not sound very much, it's still around 320KB per image, certainly enough to be useful. And if you're compressing very small PNGs then the percentage drop can seem much more impressive. We pointed the program at our 50 test PNG web graphics, and it cut their size by 40%, which could noticeably improve web page download times.
If you do have any problems with the compression, or would like the program to work a little differently, then there are some options available. Right-click the PngOptimizer window, select PNG Settings, and you can decide which parts of a PNG file are kept, and which get removed (textual data, background colour, physical pixel dimensions, more).