Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
draw.io - Online diagramming application
Mockup Generator - Create attractive product screenshots with just a few clicks
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.
Previewed - Beautiful mockups & graphics for your next app
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.