As many have told you already, a ping isn't really the right tool, as some network interfaces have ping disabed (for security reasons and also to prevent DOS attackes); also, a ping won't tell you if the web server is running. If I can stretch a metaphor here, pinging is like calling someone's house and getting the (plugged into the wall) answering machine. What you really want to do is make sure the web server is returning valid content, or inside my metaphor, reach one of the folks in the house and speak to them. They are not quite the same thing.

From the *nix command line, you can use LWP::Simple and do a one-line script, or you can even use GET to get one (or more) of the pages from the site.

Finally, I can highly recommend Nagios as a monitoring tool if you want to jump up a level to an industrial strength method.