Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
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.
Pingbreak - Pingbreak is an original and free monitoring service.
UptimeRobot - Free Website Uptime Monitoring
DownDetector - A simple service to tell you whether a website is down.
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.