Popular Mechanics debunks some of the persistant 9/11 conspiracy theories and myths. And here is another site that lists articles that disprove many of the lunatic theories.
The term "paranoia" refers to both a specific diagnosis (Paranoia or Delusional Disorder) as well as to a personality style and trait, characterized by guardedness, hypersensitivity, isolation , and suspiciousness. The individual or group exhibiting paranoia also tends to use the psychological mechanism of "projection", were they exhibit a pattern of disowning uncomfortable feelings and attributing them to others.
People or groups with paranoid disorders often have a variety of conspiracy theories, and these theories are always culturally and era-dependent. When I was first starting out as a resident in psychiatry, Patty Hearst was kidnapped by the Symbionese Liberation Front (SLF) and most of the paranoid patients I saw had delusions related to that event. Noone has them anymore, however. Nor do they have complex delusions about George Washington or Thomas Jefferson (although, in those president's days I'm sure there were many who incorporated these powerful and famous individuals into their particular theories).
9/11 has also become a focal point historically for many conspiracy theories. Other recent conspiracy sitings involve President Bush (who is either an evil genius, or a complete idiot controlled by others--like Cheney, or Big Oil etc.); Haliburton; AIDS (a disease deliberately developed to wipe out Blacks and/or homosexuals--depending on the particular paranoid); Israel (an always popular favorite--you wouldn't believe all the power they have over the world!)and of course, the vast Right Wing Conspiracy to denude the environment, destroy the weak and powerless, and undermine the foundations of science and truth.
Of course, there are always the dependable "standbys" for conspiracy theorists: the US Government in general; the CIA; Aliens (extremely popular during the run of "The X-Files" on TV) and the infamous "Trilateral Commission"--or some other secret association that is out to control the world (e.g., the Jews and the Protocols of the Elders of Zion).
Believe me, I've heard them all.