Appwrite

Appwrite is an open-source backend server that generates REST, GraphQL, and realtime APIs. It includes user authentication, a built-in database, file storage, and serverless function execution, all self-hostable or on Appwrite Cloud. The development experience is clean and well-documented.

It appeals to developers who want to own their infrastructure or need to avoid vendor lock-in. The open-source model lets you audit the code and self-host for free.