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.
JavaMelody - JavaMelody is a powerful full tool that is used to monitor Java and Java EE application servers in production and QA environment.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Grafana - Data visualization & Monitoring with support for Graphite, InfluxDB, Prometheus, Elasticsearch and many more databases
Better Uptime - We call you when your website goes down
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.