HttpWatch - HttpWatch: The ultimate HTTP Sniffer for Chrome, IE and iOS
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.
Website Speed Test by Pingdom - Pingdom is a website performance monitoring software.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Yellow Lab Tools - Website test that checks performance and HTML, CSS and JavaScript quality.
Better Uptime - We call you when your website goes down