Bay to Breakers, or Bare to Breakers for those who do it nude, is a treasure trove of weird that regularly draws twice as many participants as the New York City marathon. Although some runners take it seriously, the atmosphere of the race is somewhere between a run-of-mill 12K and the Love Parade, with people dressed in everything from stormtrooper costumes and Revolutionary War period attire to shoes...just shoes.
Unique to Bay to Breakers is the Centipede division, where groups of 13 race in a pack tethered together by rope. Sounds like a joke, but the winning centipede in 2012 finished in 36:44.
More Information: Anyone who wants to run the Bay to Breakers can enter. The race is known for having a high number of unregistered runners, or "bandits." [Third Sunday in May; zazzlebaytobreakers.com]