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.
PushBots - Light SDK for mobile push notifications
iZooto - Engage your mobile and desktop web users with intelligent web push notifications.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
PushCrew - Push Notifications for Websites
Batch - Batch is an intelligent push notifications platform for mobile apps.