Treo - Treo is the page speed monitoring tool that helps your customers from obtaining a faster web experience.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
perf - Perf is a simple app monitoring solution paired with meaningful alerts.
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.
Prometheus - An open-source systems monitoring and alerting toolkit.