Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

Supabase - An open source Firebase alternative

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

PubNub - PubNub is a real-time messaging system for web and mobile apps that can handle API for all platforms and push messages to any device anywhere in the world in a fraction of a second without having to worry about proxies, firewalls or mobile drop-offs.

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.