In this paper we describe the motivation, design and evaluation of StepGreen, a site intended to promote energy-saving behaviors. The site displays information and suggestions to an individual and his/her social network via a MySpace plugin. We first use HCI techniques including surveys, interviews and iterative design to specify the central components of the site: a set of recommended actions, visualization of CO2 and monetary savings, personalization, and a MySpace plugin. We then describe the technical details of the implementation followed by the design and results of a field trial with 32 participants. Participants found features of the site easy to use, for the most part, and appreciated personal feedback on the effects of their actions. We conclude with a discussion of lessons learned and plans for the next release of StepGreen.