POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Invision - Prototyping and collaboration for design teams
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
AppTaster - Share mockups and wireframes with clients on their iPhone or iPad.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.