Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Fuse - Fuse is a UX tool suite for mobile app designers and developers.
Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
GoodBarber - GoodBarber is a mobile app creation platform.
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.
Bizness Apps - Create your own app or become a reseller and build apps for others