Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
BuildFire - BuildFire is the easiest way for small businesses to build a mobile app in a matter of minutes for iOS and Android.
Invision - Prototyping and collaboration for design teams
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.
Sketch - Professional digital design for Mac.