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

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

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.

Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.