MontageJS - MontageJS framework is a platform to develop and create single page applications (which load single HTML page and dynamically update the app as the user puts in inputs or requests or interacts with the page.) with ease.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Platforma for iOS - The ultimate wireframe kit for app prototyping
Icons8 Lunacy - Free graphic design software with built-in resources. Fully compatible with Sketch and works offline
Platforma - Wireframe library of layouts and UI elements
Flinto - The prototyping tool designers have been waiting for.