Open Source Software (oss) is an innovative method of developing software applications that has been very successful over the past eight to ten years. A number of theories have emerged to explain its success, mainly from economics and law. We analyze a very large sample of oss projects and find striking patterns in the overall structure of the development community. The distribution of projects on a range of activity measures is spectacularly skewed, with only a relatively tiny number of projects showing evidence of the strong collaborative activity which is supposed to characterize oss. Our findings are consistent with prior, smaller-scale empirical research. We argue that these findings pose problems for the dominant accounts of oss. We suggest that the gulf between active and inactive projects may be explained by social-structural features of the community which have received little attention in the existing literature. We suggest some hypotheses that might better predict the observed ecology of projects.