Invision - Prototyping and collaboration for design teams
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project