iWebKit is a free framework designed for the creation of iPhone and iPod touch compatible websites and webapps. It provides all HTML, css and images, which allows you to create user interfaces with native iPhone app look and feel.
It is a very neat and simple solution, much better than some of the more complicated ones using jquery / ajax that I have tried. As an added bonus, it is compatible with other devices, so it will render properly on other smartphones (but will still have iPhone look).
The package includes all HTML, css and image files needed for the UI. It also includes very good documentation, and very good examples that cover all aspects of iPhone interface. It also allows things like defining a startup splash screen (if you use option to install as an app in Safari), hiding address bar and defining an icon.
I have used iWebKit with ASP.Net to create a mobile app, and it was a breeze to create different app screens. Using only provided examples I was able to create 10 different screens needed for the app very quickly and easily.
Overall a great framework which makes creating UI for webapps very easy. Highly recommended for all webapp developers.