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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
UptimeRobot - Free Website Uptime Monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations