FreeSiteWatch: Is Your Website Available?
FAQ

What happens if an error is found on my site?

If an error is found on your site you will be sent an email with some information as to what the problem might be. The email will be sent to the address(es) you have listed in your notification area of the control panel. If you opted to have your site monitored but failed to put the required link back on the home page of your site you will get a reminder email each night until you do so or until you remove the site from your monitoring list.