draw.io - Online diagramming application
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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.
Hirundin - Helping design, product and UX teams understand users and make decisions based on their needs and goals
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
ConfirmKit - User research platform for teams and product people