iPhone App Prototyping With HTML-CSS-JS: Ratchet
It has lots of iOS-looking components like buttons, forms, bars, lists, toggles, sliders, etc. that can be inserted to the prototypes with few lines of code.
The download package is just the template files and the components library. Creating layouts is very easy and their look can be customized with CSS.
Ratchet is very well-documented and has lots of new features to arrive in the future.