From the makers of Bootstrap comes a quick way to prototype iPhone apps.
An interesting technique: print out the site, cut out each component and arrange the pieces to construct the small screen view.
Jetstrap is an interface builder for Twitter Bootstrap. This looks like a tremendously helpful responsive prototyping tool as it enables less code savvy folks to drop in and modify elements to their liking.
The team at Zurb have implemented the Off Canvas Layouts by Jason Weaver and Luke Wroblewski.