Mind Map by RealtimeBoard - More than your average mind map
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Platforma Flowcharts - Collection of cards for website and app prototyping
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Wireflow - Create beautiful flowcharts
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.