draw.io - Online diagramming application
Invision - Prototyping and collaboration for design teams
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.
MockupCreator - MockupCreator is a tool to create mockups, prototypes and wireframes.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Amplitude - Mobile analytics: come with questions, leave with answers