Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
SigNoz - Open source alternative to Datadog
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Librato - Your monitoring infrastructure in the cloud. Store, visualize and alert on all your metrics at all levels of the stack. Get your 30d free trial.
Better Uptime - We call you when your website goes down