Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Intermapper - Intermapper maps your network topology, giving you a live view of what's happening on your network.
Better Uptime - We call you when your website goes down