iOS Design Kit - The newest library of native iOS templates
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
iOS Design Course - The Complete iOS Design Course with Sketch from Bitfountain
draw.io - Online diagramming application
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.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.