Not so much a sport as a joke, the Cardboard Tube Fighting League is basically LARPing for people who don't feel like sewing moons and stars on their robes. A group of people get together with a bunch of cardboard tubes and then they "duel" using loose rules about what you can and can't do with your tube. According to the official rules, you can only swing your tube at the opponent's body and can only block attacks with your tube. On the other hand, you can't use a shield or get laid while participating in this sport.
See the official site here.