Freeplane - Freeplane is a powerful and free software for building mind maps.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Visual Understanding Environment - Visual Understanding Environment (VUE) is a mind-mapping and concept-mapping tool.
OmniOutliner - OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.
FreeMind - FreeMind is a premier free mind-mapping software written in Java.
Pencil Project - Single-user mockup / wireframing / diagramming tool