Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
Happy Apps - Uptime Monitoring Simplified | Happy Apps
Pingbreak - Pingbreak is an original and free monitoring service.
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!
DownDetector - A simple service to tell you whether a website is down.
AppBeat - AppBeat is reliable website monitoring service that alerts you when there is a downtime or failure. It also measures performance and provides functional tests.