Pushover provides a notification service that can be integrated with applications.  Notifications can be sent to Android and iOS devices, including smart phones, tablets and watches.

 

Once you open a free account with Pushover you can send up to 7500 messages per application per month.

 

You will need to download the Pushover application for your iOS or Android device.  Pushover offer a 7-day trial.  After that you will need to purchase the application if you want to continue to use it.  Fortunately the cost is only USD$5 per platform (iOS or Android).  This means that you only pay for the first device you install the Pushover application on.  It is free for all other devices on the same platform, where they are all your devices or employees of your business (as long as they are all under the same account).