FiftyThree introduced the Pencil a while back. It's an advanced stylus for iOS devices with a pressure-sensitive rubber tip, an eraser on the end, a USB charging port and a direct bluetooth connection that adds amazing features to connected apps.
Up until now, the only app that the Pencil really worked with was FiftyThree's Paper. The SDK that came out today allows any app to harness the power of this unique stylus.
The SDK also means that apps can be created to do things other than drawing and painting. Touch classifiers open up interaction possibilities beyond simple multi-touch, adding the pencil and eraser tips to the available touch types. Apps can also take advantage of the Kiss-to-Pair feature found in Paper, allowing instant bluetooth connections just by holding the tip to a point on the screen. Palm rejection is improved, and with iOS8, additional input is available from the Pencil. You can determine whether the Pencil is using the tip or the broad edge, and vary the response based on pressure as well.
Some of the new apps being released in conjunction with the new SDK are impressive. Procreate, a sketching and painting app for iPad, will have Pencil integration with its three key features: paint, smudge and erase, combining them into one easy-to-use tool.
Noteshelf, a note-taking app for iPad, will allow use of the Pencil for note taking, taking advantage of its superior palm rejection technology for easy handwriting and erasing of notes.
Squiggle, a very clever music app for iPhone and iPad, will add a new dimension using the Pencil that allows you to quickly draw strings and pluck them with your finger, and quickly cut them using the eraser.
I own the Pencil and really enjoy using it with Paper. I'm anxious to see it incorporated into more of my favorite apps, and curious to see what applications that aren't necessarily drawing-based find innovative uses for it. If you're a developer interested in incorporating the SDK, head to the SDK page and click "Request Access" for more info!