This is the belief that those trails of vapor and ice crystals you see in the sky behind an aircraft is really a bunch of harmful chemicals (including metals like aluminum and strontium) being dispersed for any number of reasons (ranging from controlling climate to controlling the population). I especially want to smack the population control types. Obviously, slowly poisoning the entire atmosphere doesn't include the air our would-be overlords are breathing. Hence, it's an excellent method of controlling the masses.
This is the belief that HIV was engineered in a sinister plot to create a killer virus, and then tested successfully in Africa. Despite evidence to the contrary, some still believe it was actually first released in the US to kill homosexuals. Most of these theories put the creation of HIV somewhere in the 1970's, though blood and tissue samples in storage from as early as 1959 have tested positive for HIV. More reputable sources believe HIV is a mutation of SIV, which has been around since at least the 19th century, and estimate that HIV probably first entered the human population in the 1930's. On a related note...
AIDS is not the result of HIV
This is the belief that AIDS is caused by illegal drug use or homosexuality. Tell that to all the babies who contracted HIV either during birth or from breastmilk and then died from AIDS as young children.
Global Warming is a Hoax
The belief that the whole global warming thing was made up by the government as a grand scheme to raise taxes and enforce controls on our lifestyles. Not even the old Soviet Union would try to pull off a scheme like that. The increased temperature is well documented.
Fluoridated water dumbs down the population
Because what every developed country wants to do is try to lower the mental abilities of the entire urban population. An urban population that includes all the people running the country and supposedly came up with the scheme in the first place. By the way, after half a century of fluoridation, it looks like the small amount of fluoride only reduces the incidence of tooth decay and increases the incidence of dental fluorosis in youths (white splotches on the teeth).
The World Trade Centre collapses were an inside job
The belief that the three buildings that collapsed were a controlled demolition. This theory is unfortunately given credibility by people claiming to be architects, engineers, and demolition experts. They say that the buildings fell straight down, just like a controlled demolition. The collapse was not quite so perfect and the dust cloud was much larger than you see in a typical controlled demolition. Uncontrolled fire compromised the whole section, not just a single corner of the building. As beams and columns failed, load paths changed, other beams and columns tried to accept more load. Then they failed too...and when enough of them had failed, progressive collapse started. Then gravity did its thing: pulling things straight down. Believers of the inside job theory like to point at other building fires and say "those didn't collapse" (the Windsor Tower in Madrid is a popular one, which survived a 26-hour fire). Well those buildings weren't struck by large passenger aircraft and their structural and fire designs are often totally different (Windsor Tower wasn't even a steel-framed building, and the parts that were supported by steel columns did collapse after less than 3 hours). NIST conducted a thorough investigation and published a voluminous report that even included answers to some crazy alternative explanations. The collapses were successfully modeled using a computer program. If you watch the videos closely you will find that progressive collapse initiated by severe fire at the damaged levels is what brought the towers down. The rubble revealed that the only parts of the buildings damaged by blast were from the floors involved in the aircraft collisions.
The Moon Landing was Staged
The belief that the moon landing in 1969 was filmed in a television studio, probably just to try and make those commie bastards in the USSR feel inadequate during that international dick-waving contest we call the Cold War. One of their arguments is that, despite advances in technology, nobody's gone back since 1972. Well, round-trips to the moon are still extravagantly expensive, and still entirely useless. The "soil" and rocks brought back contained very common minerals. Basically, the moon is just a big, cratered, dusty rock that you can't even mine for valuable metals or gemstones. The Americans had put a man up there first, so the Soviets quickly gave up. After nothing more could be learned from exploring the moon, there was no reason to go back. We'll probably go back when we find a way to make it safer and less expensive.
Reptilians Rule the World
The belief that reptilian shape-shifters from the Alpha Draconis star system live on Earth among us, including the British Royal Family and former U.S. president George Bush. People who believe this also believe that most of the reptilians are living in hiding, inside the hollow Earth.
The belief that fried chicken like Church's or KFC is really a white supremacist plot to wipe out the black people. Apparently the chicken is laced with a drug that only affects black people (because that's how drugs work). Fried chicken tastes good because it's full of salt and fat. Irrespective of gender or race, eating too much of it leads to obesity and associated health problems, but not sterility.
Vaccinations are Linked to Autism/a Means to Implant Tracking Beacons/a Western Plot to Kill or Sterilize Children
The belief that vaccinations have significant health risks, or worse, are a means of controlling or experimenting on us on a massive scale. Apparently, vaccinations are a great way to implant us all with tracking beacons and test experimental warfare and mind-control techniques. Beliefs like this are dangerous because people who don't get vaccinated can put the whole population at greater risk and burden the healthcare system when they get sick. For years, polio and smallpox were on the verge of being completely eradicated, but every time volunteers would go to the third world to vaccinate babies, a religious leader would announce that it was a Western plot to kill or sterilize their children.
The Tsunamis of 2004 were Caused by the U.S. Military
The belief that the U.S. and Indian militaries, using electromagnetic pulse technology, deliberately caused the tsunamis. Electromagnetic pulse technology. I can tell that these people lack even a rudimentary understanding of physics. A tsunami is essentially a column of water, nearly as deep as the ocean, travelling radially like the ripples in a pond. A small tsunami could conceivably be caused by the deep underwater detonation of a nuclear weapon, but nothing like the ones causing the devastation we saw in 2004. Large tsunamis need a much larger release of energy, like from an earthquake or a massive landslide (the 2004 tsunamis were caused by a massive undersea earthquake).
That is to say, it isn't 2013 right now, it's 1716. This "Phantom time" hypothesis was proposed in 1991. That's right, there was a conscious effort to just make up an entire period of history, and everyone just went along with it. Even if you don't like medieval European historians and think they were making stuff up, you still have radiometric and dendrochronological (counting tree rings) dating methods, and the historical records from China, Korea, India, and Asia Minor. The dude who came up with this ridiculous hypothesis points to the appearance of Romanesque architecture 500 years after the Roman Empire fell. Because nobody anywhere at any time has ever mimicked an older style of architecture.
Lee Harvey Oswald was Framed
The belief that John F. Kennedy's assassin was just a scapegoat. While I have to concede that it would be very difficult to fire Oswald's antiquated rifle three times in eight seconds, I don't believe that it is outside the realm of possibility. There is also no doubt that the second shot came from Oswald and was the bullet that caused all of the injuries. Though the path of the bullet seems impossible to some (earning the nickname "magic bullet"), it has been recreated using computer simulations (including a controversial one that was essentially a video game). Physical recreations have been carried out using marksmen and anatomical dummies. Obviously, with so many variables, the shot can't be duplicated exactly, but these recreations have produced results so similar to the assassination that it seems ludicrous to discount the Warren Commission Report as fiction in favour of some far more implausible explanation.