iTunes link: Sketchbook Pro - £4.99
Drawing apps are a perfect fit with the iPad's highly sensitive great-looking touch-sensitive display. It almost defies reason just how powerful sketchbook Pro is. From its layer support, to the comprehensive selection of brush tools, to the clever use of multitouch presses and gestures, it really is a tour de force of the iPad's abilities.
Obviously you need a modicum of artistic ability for it to be anything more than a passing diversion, but even used as such it's still incredibly fun. What really sets Sketchpad Pro apart from its rivals is the thought that has gone into the interface. There's no clutter, no mess, but all the tools you're likely to want are within easy reach with a few presses and swipes.
In some ways Sketchpad Pro is better than many computer-based drawing applications as there's no abstraction between what your hand is doing and what's appearing on-screen.
The £4.99 asking price probably makes this a bit too expensive to just splash out on as an idle diversion. But if you're looking to do some serious drawing on an iPad, Sketchbook Pro is the app you need.