Pingdom is described as 'Monitors uptime, downtime, and website performance in real time using synthetic and real user data, with instant alerts, customizable reports, and analytics' and is a popular Uptime Monitor service in the network & admin category. There are more than 100 alternatives to Pingdom, not only websites but also apps for a variety of platforms, including SaaS, Windows, Linux and Self-Hosted apps. The best Pingdom alternative is Better Stack. It's not free, so if you're looking for a free alternative, you could try Better Stack or Web Check. Other great sites and apps similar to Pingdom are UptimeRobot, Tianji, StatusCake and Google Lighthouse.
Very easy to deploy and actively maintained, all while offering a lot of providers to receive notifications. If you're looking for a simple solution to alert you when a service goes down, this is perfect.