The paper describes Steptacular, an online interactive incentive system for encouraging people to walk more. A trial offering Steptacular to the employees of Accenture-USA was conducted over a 6 month period. Over 5,000 employees registered for the program and close to 3000 participants wore USB-enabled pedometers; from time to time they plugged their pedometer into a computer to upload hourly step counts to a website; and the website had a range of features to encourage more walking. These features included monetary rewards which were randomly redeemable through a simple game, and a social component. We describe the system and present preliminary findings about the effectiveness of these features in encouraging physical activity.