POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Invision - Prototyping and collaboration for design teams
BuildFire - BuildFire is the easiest way for small businesses to build a mobile app in a matter of minutes for iOS and Android.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
MIT App Inventor - App Inventor is a cloud-based tool, which means you can create apps for phones or tablets right in your web browser.