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.
UptimeRobot - Free Website Uptime Monitoring
Netformx - Netformx helps Cisco partners & Solution Providers deliver multi-vendor IT solutions quickly & effectively while improving profitability & customer success.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Super Monitoring - Affordable uptime monitoring service for webmasters and website owners. Have your website or server checked every minute and get alerted instantly.
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.