Question: Is there a web app generator that can create customizable applications with user authentication and data storage?

Backendless is a good option for low-code app development. It's got a visual UI builder, codeless logic and direct API integration for responsive web and mobile apps. Backendless also offers real-time database features, a visual schema modeler and database views for data manipulation. With 18+ third-party authentication providers, session management and customizable email and push notifications, it's got good user management abilities.

Another good option is Supabase, an open-source Firebase alternative that supports frameworks like Next.js, Svelte and Flutter. It's got a Postgres database, user authentication with Row Level Security, edge functions for custom code and real-time data synchronization. Supabase also offers storage for large files and vector embeddings for machine learning model integration, so it's good for a wide range of development needs.

MAGE is another good option, with a GPT web app generator that uses code generation to accelerate development. It supports a range of web apps, including TodoApp, MyPlants and Blog, with features like user authentication and data storage. That makes it a good option for developers who need to quickly build web apps that work.