Fluid UI : a very convenient HTML5 application for prototyping mobile interfaces in a browser (Chrome or Safari). Just a couple of minutes you can make a decent sketch.
It supports gestures, touch actions, drag-n-drops, transitions between pages, duplication of a template into several pages, iPhone and Android libraries, loading of custom images and screenshots. A link to the created design can be quickly sent to a colleague or tested on a mobile device. You can export to PDF, HTML or graphic format.