A low-budget, R-rated alien flick climbed to the top of the U.S. box office this weekend, edging out mega-movie G.I. Joe: The Rise of the Cobra. District 9 cost only $30 million to make, features no big-name stars, and was directed by a relatively inexperienced 29-year-old, Neill Blomkamp. The film's success is due in part to a viral-marketing campaign that "echoed the paranoia of the storyline," The New York Times reports. The ads on billboards and buses encouraged viewers to call a number to report "nonhuman activity." Thousands called. G.I. Joe came in second over the weekend, and The Time Traveler's Wife followed with $19 million in sales.