pngquant
A command-line PNG compressor that converts images to an efficient 8-bit palette format with alpha channel, typically shrinking files 60 to 80 percent, also available as an embeddable library.
pngquant is a PNG compressor that significantly reduces file sizes by converting images to a more efficient 8-bit PNG format with an alpha channel, often 60 to 80 percent smaller than 24- or 32-bit PNG files, while staying fully standards-compliant and supported by browsers and operating systems. Its compression engine is also available as an embeddable library, libimagequant.
It supports batch conversion of multiple files in one command and Unix-style stdin and stdout chaining. A configurable quality level, given as a min-max range from 0 to 100, automatically finds the number of colors needed to meet the target quality and can skip an image entirely if the result would fall below the minimum. Other options set a custom output extension or path, skip writing a file if conversion did not help, and trade speed against quality and consistency on a scale from 1 to 11.