draw.io - Online diagramming application
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
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.
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
lstore.graphic - Mockup Scene Creator