You could load an image up in Photoshop, reduce the color depth and fiddle with the pixel diffusion slider a bit. Or you could get Pixatool, a brilliant app completely dedicated to tuning pixelated images to the finest and most authentic details. Line and contrasts are rendered so well there's often an uncanny suggestion of hand-drawing, and the dithering smokes what mainstream painting apps offer.
It's $30, with a free-of-charge demo version. There's more examples. Artists are sharing their work with the #pixatool hashtag.
Update: check out the new version.