draw.io - Online diagramming application
Kanboard - Kanboard is a free and open source Kanban project management 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.
Kinopio - Thinking canvas for mind mapping, brainstorming, and moodboards
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.