iPhonized has a great post on some of the useful mockup tools available for creating iPhone and iPad apps. The tools I find most useful are the vector images of icons and buttons. The two listed below are the ones I’m using the most.
You can find some of these here:
MetaSpark’s Fireworks Vectors
A collection of vector iPhone UI elements for Fireworks CS3 and CS4. I find this incredibly useful for doing mockups for grabbing default buttons to modify. It’s a great base to start with and then you can take the design further.
Graffletopia Stencil Kit for Omnigraffle
Graffletopia’s stencil kit for Omnigraffle. It is for quick sketching of iPhone ideas. Use the outline for printouts so you can hand sketch as well. The symbols are meant as a check list for included features in your app development.