Uptime Robot

If you have a website, you need to know whether it's up or not. Sounds simple, doesn't it? Unfortunately, unless you have an IT staff (or are the IT staff), it's likely that you don't have any system in place to make sure your webhost is actually delivering on their promised 99.9% uptime. Website monitors have a simple but necessary job, to make sure that websites are up and reachable 24/7. If there is a problem, you will be notified immediately via SMS or email. Additionally, you may want to check your uptime statistics to evaluate webhost providers (or to argue about a bill).

Uptime Robot monitors up to 50 websites every five minutes and alerts you via email, SMS, Twitter or RSS if there are any problems. Most other monitoring services limit you to one site or only check every half-hour or hour. Very, very useful.

http://www.uptimerobot.com/