Using Firefox extensions to check what is remote site running

When I visited an interisting web site, I'd like to know
what was that site running.

I used http://www.netcraft.com/ to get the information before.
It is not convenient if you just want to know what the webserver or OS is.

Now, you can use the extensions of Firefox below to get the information:

Header Monitor & Live HTTP Headers:
https://addons.mozilla.org/firefox/575/
https://addons.mozilla.org/firefox/3829/

After installing these extensions, you can get the information from
Firefox statusbar.