IT ALL ENDS JULY 15: Seldom has a marketing slogan offered such direct insight into the Hollywood psyche. July 15 was of course the day Harry Potter and the Deathly Hallows, Part 2 opened, concluding a decade of Potter movies with, as it turned out, an enormous bang—$481 million in weekend ticket sales worldwide, eclipsing all box-office records and leaving studio executives gasping for adjectives. Clearly the slogan worked. But the end of a $7 billion movie franchise for Warner Bros. was hardly the end for fans. In fact, you could just as well say "It all begins July 31"—because that's the day fans can register for Pottermore, the aptly named participatory Web site that J.K. Rowling announced last month.