draw.io - Online diagramming application
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Invision - Prototyping and collaboration for design teams
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.