POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Invision - Prototyping and collaboration for design teams
BluePrint UI design - Blueprint is an UI design application for iPad and iPhone
Zeplin - Collaboration app for UI designers & frontend developers