Floyd-Steinberg - Maps colors using Floyd Steinberg Error Diffusion dihthering. The Floyd Steinberg algorithm produces excellent results in most situations and is fairly cheap, performance wise, to use.
Stucki - Maps colors using Stucki Error Diffusion dithering. The Stucki algorithm is slightly slower than Floyd Steinberg, but produces almost similar results. Is is commonly used for monochrome dithering.
Sierra - Maps colors using Sierra Error Diffusion dithering. The Sierra algorithm is slightly faster than Stucki.