Paint Shop Pro would be the obvious pick, but it only supports 5x5 ordered dithering and a bunch of other not-so-neat noisy methods. I'd like more control over dithering, like a 2x2 dithering block or more options like that.
Do Photoshop or GIMP do what you need? Gimp is OpenSource and Free, so that would be my first pick. Especially since Adobe is trying to migrate CS to "the cloud".