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.
Cachet - Cachet is a free, open source status page for your API, service or company. Built with all of the features that you'd expect from a status page.
UptimeRobot - Free Website Uptime Monitoring
cState - 🔥 Free & open source under MIT!
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Status.io - Status.io is a platform for system status pages and status notifications. Companies rely on Status.io to track outages, schedule planned maintenances, broadcast status notifications, and much more; and their customers love the transparency.