Image Alchemy [1]
Unfortunately not longer available. But one of the greatest image conversion software I know (including most of the here described algorithm for dithering and some more). And this feature set is from at least 25-30 years ago.
I'm always getting a bit nostalgia when I see these dithered images and remember the time then with the fascination for the opposite direction: 'true color... wow that would be great' ;)