When you're designing a new app, you can figure out how the app will look and work by creating mockups of each screen. This can be as simple as drawing sketches on paper, or you can use a number of software tools to create a digital mockup.
iPhone Stencil Kit - a stainless steel stencil for drawing iPhone interface elements.
iPhone & iPad Sketch Books from App Mockup Tools
iOS App Icon Template for Photoshop
MockApp offers templates for Keynote or PowerPoint
idev101's App Design Templates for Photoshop and print
AppCooker is an app for designing app mockups.
Pop App lets you take pictures of your app sketches and create a working demo on your iPhone.