App to know when sites are fully functional again after outages

After how many times Shopify went down over the holidays, I was thinking it should be an app that you can install allowing you to get notifications when a site is finally up again after an outage.<br><br>Even a simple thing like a browser plugin would be fine, but generally sites would have to be able to receive notifications when someone wants an outage notification, just like any news site or blog would offer notifications.<br><br>There are probably many out there but I generally had the idea and decided to put it out there just in case it's been overlooked.