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.
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Angle 2 Mockups - A giant Sketch Library for creating app presentations
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Balsamiq Mockups 3 - New public beta for the popular prototyping tool
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.