draw.io - Online diagramming application
Mediamodifier - Create beautiful designs and product mockups in seconds.
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.
Mockuper - Instant Mockup Generator
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Sketchize - Ready to print wireframing sketch sheets 📱