This is a subject that I've been thinking about for a while. I don't normally write about conspiracy theories, for two reasons: 1) I don't believe in any particular conspiracy theory and so has no reason to promote one and 2) There are so many more interesting and useful things to talk about.
Now I'm going to make an exception from my own rule, which I am sure I'm going to regret once the comment field gets invaded by crazy libertarians and similar.
There just isn't time to discuss all conspiracy theories, but I will explain the theory that made me write this post: the "Debt virus hypothesis", or "Money as debt". Basically, this was a conspiracy theory invented by a plastic surgeon (ever noticed how none of the conspiracy theorists are experts in their fields?), stating that the monetary system is unsustainable due to the fact that money can be created out of thin air, and then lended out for interest. Their most basic argument goes something like this: Start in a world with no money. A banker creates 1000 dollars, and lends it out for 1 year, charging 10 % interest. Now, how is the poor borrower going to be able to pay back the money? Remember, there's only 1000 dollars in the whole economy, but he owes the banker 1100 dollars. Therefore, at the end of the year, he'll have to take another loan and then another one, just to keep paying off the interest. There will never be enough money in the system to pay off all debt.
This, of course, is confusing the difference between real and nominal interest rate. The nominal interest rate is 10 %, yes, but what the banker really cares about is that he gets his 10 % in REAL interest.