draw.io - Online diagramming application
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
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.
Invision - Prototyping and collaboration for design teams
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Zeplin - Collaboration app for UI designers & frontend developers