I have produced a simple script that will let you check your sites are up and running.

The PHP script is hosted on GitHub at http://github.com/kulor/siteup/tree/master. You can see the output at http://carbonsilk.com/sandbox/siteup/

To get this script running, you will need to change some of the defaults found in index.php and place both the files in a web accessible directory on your server

Automating the script

The best application of this script would be to deploy on your server (not on the server you are checking is up) and run on a crontab at a fairly liberal interval (2 hours) to save server overload. To do this:

On your command line:
$ crontab -e

This will take you into an editor window. Enter the line below replacing {$path} with the location of your script
0 */2 * * * php -q /{$path}/siteup/index.php

Now you will receive an email if any of your programmed domains are down.