JSON Generator - Create mock and sample JSON using a powerful template syntax
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Mockaroo - A realistic data generator to test your app
Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
Random Data - Generate random data for testing
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.