Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
UptimeRobot - Free Website Uptime Monitoring
UptimeRobot App - Web, SSL, ports & Cron-jobs 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.
Curler - Dead Simple Free Uptime Monitoring