

Prototyping, Mockup, and Wireframing Tools for iPhone App Development - iphoneized
http://iphoneized.com/2009/11/21-prototyping-mockup-wireframing-tools-iphone-app-development/
It's far from being the definitive list, but it's definitely a good place to start if you are creating an iPhone app mockup.
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grinich
What about Interface Builder? It's free, and your prototypes just become the
UI.

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xinsight
Is it possible to not end up with a generic and dull UI with IB?

