Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
AppMySite - An automated no-code app builder
Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
AppYourself App maker - App maker designed by AppYourself is a popular app builder platform that gives you the opportunity to create feature-rich Android, iOS, and PWAs apps in minutes.
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.
Jasonette - Make an app by adding JSON to this app.