JAVA Webpage monitor

Hi All
I am looking for a simple Javascript that I can embed in a single webpage
that will monitor several websites to ensure that they are active.
I beleive the javascript would have to connect to the specified website and
check that it could access a sertain line of text from that website. IF it
does this ok then I need it to diplay a graphic (Green icon) if it cannot
get the text because the webpage is unavailable or the text has changed then
display a red icon.
If anyone can point me in the right direction it will be very much
appreciated.
Regards
Mike