UptimeRobot - Free Website Uptime Monitoring
Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
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.
Silent Down - Silent Down is a website uptime and performance monitoring platform - Get notified instantly if your site is down or having performance issues.