If you use Google to ask this question the first hit is actually the pngnq website, which explicitly states that pngnq is a modification of pngquant, and supposedly an improvement.

http://pngnq.sourceforge.net

> Pngnq is an adaptation by Stuart Coyle of Greg Roelf's pnqquant using Anthony Dekker's neuquant algorithm.

> Why another quantizer?

> Pngnq exists because I needed a lot (several thousand) of png images in RGBA format to be quantized. After some searching, the only tool I could find that worked for RGBA was pngquant. I tried pngquant but found that the median cut algorithm that it uses, with or without dithering, gave inferior looking results to the neuquant algorithm. You can see the difference demonstrated on the neuquant page.

It produces same type of file PNG8, but:

* pngquant has adaptive dithering which compresses better and makes less noisy images

* pngquant has more predictable algorithm and usually gives better quality (pngquant aims for minimum square error and guarantees local optimum, while pngnq has neural net algorithm that sometimes works well, sometimes misses details or generates useless palette entries)

* pngquant can automatically choose number of colors required for desired quality and leave 24-bit image if it requires too many colors.

* pngquant's algorithm is available as embeddable libimagequant