This paper is an initial exploration of the determinants of open source project success as measured by project popularity. We simultaneously model the impact of project-specific characteristics on project popularity, and the impact of intended users and choice of operating system on the choice of end-user license. These models are jointly estimated using Full Information Maximum Likelihood Method. The results show that the software-user license, age of the project, project status, certain types of potential users, and compatibility with certain operating systems have a statistically significant impact on project popularity. An interesting finding is that GPL, the most widely used software license has an adverse impact on the popularity of an open source project.