POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
quickMockup - Online tool for creating mockups in the browser.
Briefs - Briefs is the essential app design tool built for a professional design workflow.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Invision - Prototyping and collaboration for design teams
AppTaster - Share mockups and wireframes with clients on their iPhone or iPad.