Flutter is described as 'Build apps for any screen. Flutter transforms the app development process. Build, test, and deploy beautiful mobile, web, desktop, and embedded apps from a single codebase' and is a popular app in the development category. There are more than 25 alternatives to Flutter for a variety of platforms, including Mac, Windows, Linux, Web-based and Android apps. The best Flutter alternative is Android Studio, which is both free and Open Source. Other great apps like Flutter are Qt, Electron, React Native and GTK.
I like Titanium Mobile because you can use normal JavaScript to write your apps and Alloy (MVC Framework) makes your app very structured and easy to use. It's fully open source and well tested. Easy to extend with native Android or iOS modules