What are some strange conspiracy theories you have heard of?

One of the earliest conspiracy type theories I heard when I was younger concerned Gumby and Pokey. They were an advance force of aliens looking to invade our world. The proof was that pretty much everyone had a Gumby or a Pokey at one time or another when they were young. And except for the few that the dog chewed up, no one could actually remember what happened to them, but they are all gone. They had all gone back to the mother ship to report. Yes, this theory did involve recreational pharmaceuticals. Are there any other bizarre theories out there? The weirder, the better!