Optipng

OptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information. OptiPNG offers roughly the same functionality as pngcrush, while running faster. This program also has additional features, such as convertion from external formats (BMP, GIF, PNM and TIFF). See the home page on sourceforge.

The general subject of PNG optimisation and tools is discussed more in depth in this article: A guide to PNG optimization.