Freeplane - Freeplane is a powerful and free software for building mind maps.
Open Peeps - A hand-drawn illustration library.
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.
Control Illustrations - 108 free flat illustrations with customizable characters
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
DrawKit - MIT licensed SVG illustrations, in 2 styles