In the US we call those folks who believe in all kinds of conspiracies especially those about the government as the "black helicopter crowd".
I am sure most of you have heard of theories with regards to the events of 9/11 ... the Jews were behind the plane crashes on the twin towers, the Bush administration blew up the Pentagon themselves, CNN did not report the whole truth about the 9/11 events etc etc. :-)
There are many sites on the internet with their theories about what "really" happened on 9/11 and there are also sites that debunk the theories. Here's one site that has the most loony theories put together on one page. :-)
The most controversial of the events of 9/11 is on what actually happened to the Pentagon. I found the following video clip via Little Green Footballs. Anyone with some shred of common sense can readily see for himself/herself that the computer simulation presents the most likely sequence of events that happened at the Pentagon that day.
But then again not everyone is going to readily believe this video clip as attested by the comments found on You Tube where the video is posted. There are believers of these conspiracies in places as far flung as Malaysia. So what else is new, eh?