iPhone App Prototyping With Photoshop: WireKit
Prototyping any type of design (web/app/code) helps minimizing hassle in the next steps of the creation process.
WireKit, a set of free Photoshop shape layers, simplifies creating prototypes for iPhone apps.
It comes in 2 versions: solid (realistic) and stroke (sketchy) where each version includes 60+ elements.
The layers are very well organized + named and they are all consistent in size + how they look.