Greetings
Conspiracies "exist"... I don't know wich is true and wich is not, but here is an example of a well documented one:
The Orde de Jacque Cartier, also know as "La Patente", when its member wanted to talk about it in public (patente is a french word for "thingamabob" ), was a secret society that had several thousands members in Canada, up to 11 000 at its peak. All were french speaking, and its main goal was to defend and promote the rights of the french speaking minority. It lasted for over forty years (it no longer exist... as far as I know), and had lawyers, journalists, doctors, burocrats and polititians (in the goverment AND the opposition... at the same time often) among its members.
Now, I'll admit that it isn't very sinister or anything, but for a very long time this secret society existed and manipulated events in Canada as much as it could. There wouldn't be a separatist movement (a strong one) in Quebec if it wasn't for it. Now, if you can have a secret society for "good" (the french minority was being repressed), you can certainly have one for not so good goals.
If you enjoy conspiracy theories, by the alternity seting "dark matter". It is chock full of them, and gives you a setting to play in them! Plus, it is fairly rule light... you could convert it to d20, or use the spy craft rules or the upcoming d20 modern rules to play!
Ancalagon