UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
draw.io - Online diagramming application
React Proto - React application design and prototyping tool
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.
Userflows - Automatically generate user journeys
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.