UptimeRobot - Free Website Uptime Monitoring
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
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.