libpng is the PNG image format reference implementation. Portable Network Graphics are the most widely used raster image format, patent-free, based on lossless data compression, support indexed/paletted, grayscale, and 24 or 32 bit truecolor images with transparency. It's also an highly extensible container format, with built-in color profiles and representation information, textual meta data, filters, progressive interlacing, and permits animations in the derived APNG or MNG formats.
Pngcrush is size optimizer for PNG images. It's a command-line tool, works on Unix and DOS environments. It reduces the IDAT picture data stream by testing different PNG filter methods and compression levels. It may also remove meta data chunks, or inject gAMA, tRNS, iCCP or text chunks.