Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Pushy - Pushy is a reliable push notification gateway for time-sensitive, mission-critical apps.
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.
Parse - Build applications faster with object and file storage, user authentication, push notifications, dashboard and more out of the box.
Pushmaze - PushMaze is a self-hosted Mobile and Web Browser Push Notifications service. It lets you send push notifications 100x cheaper via Firebase Cloud Messaging.
Nakama - Nakama is an open-source distributed social and realtime server for games and apps.