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.
Traq - Traq is a PHP powered project manager and issue tracker featuring multiple projects, milestones...
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
16bugs - 16bugs is an all-in-one bug tracking software that makes it easier for you to manage all sorts of bugs in performance with the simple interface; ultimately, you will be more productive for sure.
Better Uptime - We call you when your website goes down
Trac - Trac is a project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.