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.
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
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
Better Uptime - We call you when your website goes down
Webcron - Webcron - Website and Server Performance Monitoring and Online Web Cron. Reliable Cron Jobs Service with WebCron interface. The easiest way to set cron jobs.