I currently have a site, that the WordPress installation is “going squirrely.” It just pops a Internal Server Error on a regular basis (even though its not the hosting, as all I do is delete the .htaccess file, and the site comes right back up). None the less I have some Google adsense money that is currently promoting the site. As I troubleshoot what is going on, I want to know if the site is down. This is where www.site24x7.com comes into play.
There is a free version that you can use, but for next to nothing you can have your site checked every 5 minutes, and if it turns out its down you can have the system notify multiple people by email, or by texst message.
You can try it for 15 days free. The interface is super easy to use, and while I'm trustrated with my WordPress install, I am happy that I get instant notification and I can make adjustments to my site to get it back up. Its given me a lot of peace of mind. Check it out at www.site24x7.com