The "Finland isn't real, it was made up so Japan could hunt whales in the open water where it supposedly exists" one is my favourite. Avril Lavigne dying and being replaced by a doppelganger is a good second.
Do I believe any? Not that I'm aware of, lol. Maybe the one about mattress stores being a front for the mafia because apparently there's a ton of them in the US and no one can figure out how they make money, but like... with /r/AskReddit as my only source, I'm not about to go down for this ship.
Hm... probably the whole Roswell thing. A lot of people think that aliens landed here in Roswell, New Mexico. However, it's also said that the government is covering it up... which doesn't really surprise me. I'm one of those people that believe other life exists and such. I also have a belief that if there are aliens, they have no intention of wanting to hurt us in any kind of way. After all, if that was really their intention... why haven't they done it already? If anything, they are curious of us as much as we are of them.