Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
UptimeRobot - Free Website Uptime Monitoring
Pingbreak - Pingbreak is an original and free monitoring service.
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.
DownDetector - A simple service to tell you whether a website is down.
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