UptimeRobot - Free Website Uptime Monitoring
ApicalSoft - Radar Website Monitor is a multithreaded website monitoring tool, watching different protocols, including HTTP, SMTP, POP3, FTP, TCP, DNS and Database.
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.
SpringTrax - How do you find the 404 errors on your website? Once found, how do you fix the 404 errors? What tools can help? Find out in this in-depth 404 guide!
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