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.
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.
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.