iPhone prototyping tools
could anyone suggest some good (either free or commercial) iPhone / iPad apps prototyping tool?

I'm interested in creating app mockups aimed to both supporting design + development and displaying app stubs in business plans. 

Thanks.
iPhone prototyping tools
===

could anyone suggest some good (either free or commercial) iPhone / iPad apps prototyping tool?

I'm interested in creating app mockups aimed to both supporting design + development and displaying app stubs in business plans. 

Thanks.