Anyone who manages mission-critical (or even vaguely important) websites knows the need to be on top of downtime awareness. On your PC you have lots of choices: Pingdom.com springs to mind, especially with their Personal account letting you monitor up to 10 sites for free.
Overlook Wiz. It's widget (or several, if you monitor many sites) that sits on your home screen, and alerts you when the site is unreachable.
You can set the interval at which it checks the server, and you specify a specific URL to check. Note that a HTTP check looks for a specific word on a page, so be careful what word you specify.