iOS Design Kit - The newest library of native iOS templates
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
iOS Design Course - The Complete iOS Design Course with Sketch from Bitfountain
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Xcode - Xcode is Apple’s powerful integrated development environment for creating great apps for Mac, iPhone, and iPad. Xcode 4 includes the Xcode IDE, instruments, iOS Simulator, and the latest Mac OS X and iOS SDKs.
BluePrint UI design - Blueprint is an UI design application for iPad and iPhone