Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
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.
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.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.