OptiPNG: Advanced PNG Optimizer
OptiPNG is a PNG optimizer that recompresses image files to a smaller
size, without losing any information. This program also converts external
formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity
checks and corrections.
If you wish to learn how PNG optimization works, or to know about other
similar tools, see the PNG-Tech article
A guide to PNG optimization.