Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
JSON Generator - Create mock and sample JSON using a powerful template syntax
Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
Mockaroo - A realistic data generator to test your app
Kodular - Much more than a modern app creator without coding
Ideas Are Worthless - Like Product Hunt. But for Ideas.