ApicalSoft - Radar Website Monitor is a multithreaded website monitoring tool, watching different protocols, including HTTP, SMTP, POP3, FTP, TCP, DNS and Database.
PingTurk - PingTurk is a cloud-based platform to monitor website from many locations and behind different networks, reporting on customers experience.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue