FiftyThree now has a software development kit available for devs that want to make use of their snazzy Bluetooth-enabled Pencil accessory. This enables new interactions whereby the app can recognize a fingertip, a palm (which can generally be rejected as input), the tip of the Pencil, or the rectangular eraser head.
There's also some super-simple pairing functions available for those that bounce between devices, plus once iOS 8 drops, Pencil will support pressure sensitivity. Three top-notch developers are already making use of the tools, including Procreate ($5.99), Noteshelf ($5.99), and Squiggle ($0.99).
Interested developers can hit up the Pencil SDK page to sign up. If you don't have a Pencil and think you could get some use out of it (especially with forthcoming third-party app support), you can snag one below. Obviously Pencil works great with FiftyThree's award-winning Paper app, which you can also try out at the download link below.