UptimeRobot - Free Website Uptime 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.
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.
Apex Ping - Uptime & performance monitoring for websites and APIs.