IP Sentry can also be programmed to email you if your site goes down.
In the old days you could "ping" a web site. Now for security reasons this is turned off pretty much everywhere. What IP Sentry does, from their site, is:
IPSentry actually issues a GET request to the server and inspects the returned data to ensure that the data that was received contains the expected information. The pre-defined entries utilize the data send/receive information based on published RFC specifications.Do I need to monitor my site? Not necessarily, we just like to point out that you CAN if you want to.
Disclaimer stuff - those are not affiliate links and we have no relationship with IPSentry at all. They just have a cool reasonably priced network monitoring utility.
Another great resource for those REALLY into network monitoring is the Open Management Consortium.