RemoteHQ - Real-time collaboration for distributed teams
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
Diagrams for Mac - A diagram editor for visualizing structures and processes
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Stately Viz - Visualize, simulate, and inspect your state machines
Freeplane - Freeplane is a powerful and free software for building mind maps.