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.
Freeplane - Freeplane is a powerful and free software for building mind maps.
draw.io - Online diagramming application
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.