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.
UptimeRobot - Free Website Uptime Monitoring
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources