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.
StatusGator - Monitor all your hosted applications, vendors, and websites, and keep your team informed of outages. StatusGator is the world's premier status aggregator, status page, and website monitoring tool.
UptimeRobot - Free Website Uptime Monitoring
Amazon CloudWatch - Amazon CloudWatch is a monitoring service for AWS cloud resources and the applications you run on AWS.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.