ImageAlpha is a GUI for pngnq and pngquant.
It allows you to easily generate paletted PNG images with full alpha channel and easily preview them against multiple backgrounds, to find optimum number of colors.
nQuant is a .net color quantizer that produces high quality 256 color 8 bit PNG images. nQuant often reduces the size of a 32 bit image to a size 3 times smaller than its original with quality surpassing what the standard C command line utilities produce.
nQuant was developed as part of a larger
... [More] effort, RequestReduce, which is an http module that automatically minifies and merges CSS as well as sprites their background images on the fly. I wanted the sprited files to be optimized and I was not satisfied with the size of the 32 bit images that the .net library was producing nor was the quantization output of such quantizers as PNGQuant and PNGNQ of acceptable quality. I set out to create this quantizer and the results are images 3x smaller than their 32 bit originals with little loss. [Less]