In a world where Live Nation and Ticketmaster join forces to dominate event tickets, you want a tool to fight back. SeatGeek projects where the secondary market for hot tickets is heading — the same way Bing Travel predicts if airfares are going up or down — then recommends the best time to buy and the cheapest rows for your next ballgame or mega concert. The free service aggregates prices from StubHub and other ticket marketplaces and compares them, using an algorithm that considers historical trends and such variables as team records, venue capacity, and number of tour dates. "We felt constantly that we were being ripped off," says cofounder Jack Groetzinger, 25, who stumbled upon the idea in Boston dealing with the Fenway frenzy over Red Sox tickets. SeatGeek boasts an 85% accuracy rate and takes 7% to 10% from ticket markets for the sales traffic it drives. In a $15 billion industry, that's all-star potential.