1. La Cosa Nostra. AKA the American Mafia. A conspiracy theory until Joe Valachi became the first Mafia member to turn state's evidence in 1963. Until then, if you tried to talk to people about it, you were a kook.
3. Gulf of Tonkin. You know, that shit that got the USA officially into the Vietnam War.
In late 2005, the National Security Agency declassified its own official history of the Gulf of Tonkin and admitted that intelligence agency officers had deliberately skewed the intelligence and claimed that Vietnamese patrol boats had attacked US Destroyers on Aug. 4th, 1964, when in reality they had done nothing even while being fired on by US forces. Exactly like the captains of the US ship had described it right after that got back into port.
4. General Motors streetcar conspiracy
5. Project MK-ULTRA
MKUltra used numerous methodologies to manipulate people's mental states and alter brain functions, including the surreptitious administration of drugs (especially LSD) and other chemicals, hypnosis, sensory deprivation, isolation, verbal and sexual abuse, as well as various forms of torture
all without consent.
that is just starters.