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.
quickMockup - Online tool for creating mockups in the browser.
Swift Playgrounds - Learn serious code on your iPad in a seriously fun way
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
Invision - Prototyping and collaboration for design teams