POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
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.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Freeplane - Freeplane is a powerful and free software for building mind maps.