Mine's gotta be the massively scoped Patriot Movement. A brief rundown:

• The entity which is considered the government of the United States is in fact a corporation established in 1860something.
• There is a real you and a fake you which is your name spelled with capital letters. The banks and Corp. US can only interact with the fake you.
• The Corp. US has been under martial law since the Civil War, you can tell because the flags in government buildings have gold tassels or something.
• Alaska and Hawaii weren't approved by the US, just the Corp. The true US has only 48 states.
• There are a bunch of forms you can file with which you can sneak your way out of the system (depatriating/repatriating, land grants, something involving the universal commerical code).

It kinda almost makes sense as an allegory or something, but people believe these things exist.

The belief that >>2 gets are inherently valuable.

The conspiracy that computers are stolen alien tech

I have this crazy conspiracy theory about crazy conspiracy theories...

During the 1960s, the USSR was developing a weaponized form of rabies. Essentially, this disease would appear as a normal fever but the victims would eventually turn crazy and start attacking those around them, spreading the illness through bites. If this had been kept secret and the populace were unprepared, the Soviets could have inflicted massive casualties by releasing the weapon in the US using a few sleeper agents. This was designed as part of a strategy to to weaken the US at home should war break out in Europe. This was an ideal solution to the inability of the Soviets (at the time) to strike the US with a crippling nuclear attack. In order to prepare the populace, the CIA covertly funded the production of zombie movies. These were superior, they figured to outright propaganda films, as they were less likely to cause a scare or to upset relations with the USSR. These embedded in the minds of Americans the idea of a plague that rendered its victims beyond control and as such, they had to be eliminated. Think about it: do you know what to do in the event of a zombie plague? Simple: Take a defensive position, arm yourself, isolate the wounded and sick. That's all you'd need to survive an outbreak of weaponized rabies. As the Soviets' abilities to strike the US with nuclear weapons improved, their studies into biological weapons diminished. However, we're still ingrained with that relic of the era in our popular culture.