UptimeRobot - Free Website Uptime Monitoring
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.
Netumo - Ensure healthy website performance, uptime, and free from vulnerabilities. Automatic checks for SSL Certificates, domains and monitor issues with your websites all from one console and get instant notifications on any issues.
Pingbreak - Pingbreak is an original and free monitoring service.
Wormly - Wormly is a website uptime monitoring solution.
Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.