PocketBase is a open-source backend in a single Go file with real-time DB, auth, and file storage. It handles single file, real-time, auth, and s3 storage, and it's best suited for developers wanting a single-file go backend. Launched in 2022, it's a relatively recent entrant that's been gaining traction.

The core product is entirely free. Since it's open-source, you can self-host for free with no user limits. It's aimed at individual users, so it's fast to set up but may lack team-management features if you scale.