OptiPNG is a free command-line tool (gui extensions are avalaible), that will help you optimize PNG files, to reduce their size to a minimum without losing significant (perceptible) information. OptiPNG will also perform some auxiliary functions like integrity checks, metadata recovery and pixmap-to-PNG conversion.
Valid PNG files that cannot be optimized by this program are normally left intact; their size will not grow. The user may request to override this default behavior.
Input files are raster image files encoded in PNG format, or even in GIF, BMP, PNM and TIFF formats.
If the image is in PNG format, the program attempts to optimize the given file in-place. If optimization is successful, or if the option -force is enabled, it replaces the original file with its optimized version. The original file is backed up if the option -keep is enabled.
If the image is in an external format, OptiPNG creates an optimized PNG version of the given file. The output name is composed from the original file name and the .png extension. Existing files are not overwritten, unless the option -clobber is enabled.
If you prefer to use the program under a graphical user interface (gui), you can download Punig (it includes OptiPNG in its own executable).