FreeMind - FreeMind is a premier free mind-mapping software written in Java.
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.
Xmind - Xmind is a brainstorming and mind mapping application.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
OmniOutliner - OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.