CatLight - Shows build status in your tray
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.
Jenkins - Jenkins is an open-source continuous integration server with 300+ plugins to support all kinds of software development
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
CCTray - Display continuous integration status from most build servers.
Better Uptime - We call you when your website goes down