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.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Better Uptime - We call you when your website goes down
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.