Monitority - Simple website monitoring service (sms/email alerts)
Uptime Doctor - You can monitor up to 5 URLs with 1-minute monitoring interval, for FREE!
Suprvise - Know instantly when your sites are down.
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.
PingPing - A simple tool to monitor your website and SSL
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring