I have a few websites on different servers, Is there anything quick and easy to setup, so those website will be monitored? I just need to know the website is accessible or not.
hyperspin.com is a pretty good cheap hosted solution.
If you want something more intense to run on your own systems check out IpSwitch WhatsUp (www.ipswitch.com). There are also a lot of these monitoring apps on SourceForge but the more cheap/free they are, the harder they are to get set up.
Simplest solution: A small script that requests a defined page (homepage?) from the webpage and checks if a specified string appears in the retreived document.
Overkill solution: http://www.nagios.org/
You could use wget. Hack together a simple script to check the sites. Nagios or Cacti are good and you could set them up later for more serious monitoring.
Something like:
wget --spider -S www.mysite.com
Then check the output for the "200 OK"
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Date: Wed, 11 Feb 2009 15:13:28 GMT
Server: Apache
X-Powered-By: PHP/5.1.2
Keep-Alive: timeout=10, max=300
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8
Length: unspecified [text/html]
200 OK
Have a look at Pingdom, it's a cheap Web service ($10 / month) and super easy to set up.
I chose a local solution to keep track of my server. This solution technically tells me if the server is up, but it does not actually tell me if the HTTP daemon on my server is running. So there are multiple levels of checking that can be performed.
On my site, I created a simple script that gets the time stamp of a dummy file and emails me if the time stamp on that file is more than 10 minutes old, then the same script touches the dummy file to force the time stamp to become the current time. Then I set up a cron job to run this script every 10 minutes.
Writing a separate script to check for the presence of the HTTP daemon would be pretty easy too. The solutions that involve downloading a page from your website cyclically would have the problem of network equipment between your client and server possibly going offline and giving you false indications.
If you're looking for a great website (and device) monitoring system, check out binarycanary.com
They offer FREE 15 minute monitoring of up to 5 web pages over HTTP or HTTPS! Plus their paid accounts (starting at $5 per month) include 1 minute monitoring of nearly any device - FTP, SMTP/IMAP/POP3, round trip email, PING, TELNET, SSL Certificates, Domain Names and more.