Many of you probably use a service like this, however for those that don't here is a service you will find useful.
Uptime Robot allows you to monitor up to 50 website's uptime for free, notifying you when a site or service is down, via email or via the API.
It's free for up to 50 sites and then quite inexpensive for more.
https://uptimerobot.com
The dashboard and tools are simple and easy to use, the feature set is limited by your imagination because they have an API. If you can't use an API just get email alerts about your sites and use the simple dashboard. Any time a site goes down you'll get an alert.