I've been on Mturk for about a year and a half with 7700 approved HITs, mostly surveys. Many of these surveys say the end a random worker will be chosen from a raffle to receive a prize. Usually $20 to $50 or gift cards. I never count on winning these or hear back from the Requestors, but I would think that with as many I have done I would have probably won at least one. I personally don't believe these raffles exist, but I curious to see if anyone has ever won.