Everyone and their brother has downloaded a brushes-based drawing app for their iPad. Those apps are fine, but if you're looking for a something outside the box, you need to check out Sktch.
Sktch was designed by several digital artists and published by CreativeApplications.Net. It includes 16 presets that let you draw circles, meshes or network-style compositions based on a blank image or a photo from your camera roll. You can even view your creation in 3-D mode and change the x, y and z-axis of your creation. The resulting drawings are angular, abstract and immensely captivating.