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
HyperSpin - HyperSpin is an animated arcade frontend for Windows for use on Home Arcade Machines.
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.
Monitis - Monitis specializes in IT monitoring, covering server, website, app monitoring and more within an all-in-one monitoring platform.