StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.
Apex Ping - Uptime & performance monitoring for websites and APIs.
Better Uptime - We call you when your website goes down
PingTurk - PingTurk is a cloud-based platform to monitor website from many locations and behind different networks, reporting on customers experience.
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.