This is a slightly modified version of pngnq, with an Xcode project file.
We've modified the command line interface to better support our workflow.
Original can be found at http://pngnq.sourceforge.net/
Usage: pngnq [-fhvV][-g gamma][-n colours][-Q dither][-s speed][input file][output file] Options: -n Number of colours the quantized image is to contain. Range: 16 to 256. Defaults to 256. -f Force ovewriting of files. -g Image gamma. 1.0 = linear, 2.2 = monitor gamma. Defaults to 1.8. -h Print this help. -Q Quantization: n = no dithering (default), f = floyd-steinberg -s Speed/quality: 1 = slow, best quality, 3 = good quality, 10 = fast, lower quality. -v Verbose mode. Prints status messages. -V Print version number and library versions. Quantizes a 32-bit RGBA PNG image to an 8 bit RGBA palette PNG using the neuquant algorithm.