Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Freeplane - Freeplane is a powerful and free software for building mind maps.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Platforma - Wireframe library of layouts and UI elements
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.