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
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue
Robodash - Cron and uptime monitoring dashboard