This paper investigates the use of open source, idea competition platforms to catalyze citizen participation. It specifically focuses on an initiative of Ashoka’s Changemakers, an innovative international nonprofit to develop this capacity to leverage modest philanthropic resources and inspire citizens to offer ideas about solutions to public problems. Drawing upon theory from collective action, we empirically examined how project design affects contribution to forty-seven projects held from 2004 to 2011. The analysis reveals the number of contributions is higher when the projects are highly visible, when specific skills are required, and when outcome measures are specified in participants’ proposals. This analysis gives support to existing theory about online engagement and considers implications for policy, practice and future research about use of and design of open source platforms in public affairs.