Free Interactive Prototyping Tool – Indigo Studio
Indigo Studio is a free tool for quickly creating prototypes of web, mobile or desktop apps.
The application works on both Windows + Mac OS X and not only capable of generating static outputs but also supports interactions (links, animations, etc.).
Usually, prototypes can look plain and customers can have difficulty in imagining how things would look in real life. Indigo Studio solves this by providing lots of storyboard scenes with people, places and various devices to create a more realistic look.
It is also possible to import hand-drawn sketches into the application and adding interactivity to them.
Indigo Studio is a pro quality yet free application and only requires a free membership to the site.