Decades of research have explored factors that can influence green behavior. However, much less is known about how technology in general, and social technologies in particular can motivate people to participate in green activities. In this paper we describe the goals, design and evaluation of StepGreen.org, a site intended to promote energy-saving behaviors. We present the results of a field study, during which participants chose to engage in different actions and reported when they had completed them. Our results suggest that motivating factors like public commitment and competition are effective, and better leveraging these factors will likely lead to even greater appeal and effectiveness. Our contribution is to create an understanding of the impact of different decisions on the success of StepGreen.org that can benefit the designers of other multifaceted, online systems for behavior change.