StatusPage.io - StatusPage.io is the best way for web infrastructure, developer API, and SaaS companies to get set up with their very own status page in minutes. Integrate public metrics and allow your customers to subscribe to be updated automatically.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
UptimeRobot - Free Website Uptime Monitoring
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.
FreshStatus - Better status pages in 1-click, FREE FOREVER
Nagios - Complete monitoring and alerting for servers, switches, applications, and services