POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Adobe XD - Adobe XD is an all-in-one UX/UI solution for designing websites, mobile apps and more.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Userflows - Automatically generate user journeys