Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources