Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Parse - Build applications faster with object and file storage, user authentication, push notifications, dashboard and more out of the box.
OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.
AWS Amplify - JavaScript library for app development using cloud services
AWS Mobile Services - Storage, Analytics, Push Notif. & SMS Delivery SDKs
Nakama - Nakama is an open-source distributed social and realtime server for games and apps.