Pixelmash is clever indeed: create your resolution-independent art with the same freeform speed as you might in any other painting app, then let it nondestructively pixelize it, with 1-pixel outlines, adjustable gradients and dithering.
Pixelmash's resolution-independence lets you do really cool things… Like create animations using layer transforms rather than having to paint every frame pixel-by-pixel. Or make outlines, shading, and dithering easily adjustable by having them applied as layer effects. Or easily create different resolutions and color variants of the same image. Or convert photos or other hi-res artwork into pixel art using layer effects and the resolution slider.
Free demo, $15 to pre-order.