UptimeRobot - Free Website Uptime Monitoring
Domain-Monitor.io - Looking for the complete domain and website uptime monitoring solution? Domain Monitor keeps an eye on your website, cron, SSL and domain name 24/7 and will alert you through Email, SMS and web when something stops working.
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.
DomainMOD - DomainMOD is an open source application written in PHP & MySQL used to manage your domains and other internet assets.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Tough Domains - Tough Domains is platform that provides tools to manage internet domains, domain insights, and domain monetization.