Fully managed database agnostic GraphQL API service with first-class support for real-time and offline.
Check out projects like Conference App in a Box which use AppSync to make comments real-time.
FaunaDB is a serverless cloud database (for OLTP) that gives you ubiquitous, low latency access to app data, without sacrificing data correctness and scale. With native support for GraphQL, FaunaDB offers a flexible data model spanning relational, document and graph models.
FaunaDB is priced based on your usage, and offers a free tier.
Cloud Firestore is a flexible, scalable database for mobile, web, and server development from Firebase and Google Cloud Platform.
Firestore is newer and Google encourages you more to use it over Firebase's original realtime database. Comparison.
Perhaps Firebase Realtime Database's most useful and iconic feature is the realtime data storage feature. Once you connect, you can not only push data to the realtime database, but watch and respond to data changes in near realtime.
Hasura is an open source engine that connects to your databases & microservices and auto-generates a production-ready GraphQL backend.
A fast, simple and self-hosted backend as a service for your (web) apps, Open Source and free. No need to write server-side code or database schemas. Makes building offline-capable software a breeze.
JSONbin.io is a free JSON storage and JSON hosting service. It is an ideal solution for storing JSON data for small-scale web apps, mobile apps, or websites.
There is also JSONbin.org, which is very similar.
Para is a flexible and cost-effective backend service that stores, indexes and caches your objects.
Sheetsu helps you connect Google Sheets to anything - Web, Mobile, IoT or any service's API.
Google Sheets has an API directly, but it's not particularly easy to use. Sheetsu makes it less sheety. Get it. That's not their tagline I just wrote that.
Also look at Tabletop.js for an open source library to help with the Google Docs API.