If you have a web site and you want to know when it goes down, you might be interested in a web server monitoring service.
Pingdom offer monitoring of a single site for free, including 10 SMS alerts. Pingdom is probably the best of breed web site monitoring tool, we use it across all our production sites, and they also offer transaction monitoring, whereby you can specify a series of actions to be taken, and various checks to verify that those actions succeeded. Smart.
If you post a link to Pingability.com on your home page, they will monitor your web site for free. Not a bad deal, but also not a great deal. Pingability offer some cool features whereby you can trigger SSH actions on failures, a sort of poor man’s monit. Only on their paid plan of course.