Amazon Simple Notification Service is a flexible, and fast messaging service which is fully managed. SNS is a simple and cost-effective method to message both distributed services and human recipients.
OneSignal is a simple and smart service that send push notifications across all platforms. OneSignal features reliable delivery of millions of notifications, segmentation and targeting, automated delivery, localization, support for all major app development tools, and real time analytics.
Pushbullet makes it easy to push notifications to your smartphone and computer from the web. These notifications are smart too: if you receive a notification with a link, tapping on the notification will take you right to it!
Horntell is a notification center which allows you to have a single feed for all your notifications. Stay updated through actionable notifications and have total control so no one can spam you.