draw.io - Online diagramming application
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
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.
Smartmockups - Create stunning product mockups, easily and online.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
MockMagic - A free, easy-to-use device mockup generator