Freeplane - Freeplane is a powerful and free software for building mind maps.
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.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
draw.io - Online diagramming application
SuperNotecard - Introducing SuperNotecard. SuperNotecard is an online writing tool that features virtual notecards to help arrange facts or scenes, track details, organize paragraphs, and clarify your composition process.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.