Design-based research (DBR) and open source software are both acknowledged as potentially productive ways for advancing learning technologies. These approaches have practical benefits for the design and development process and for building and leveraging community to augment and sustain design and development. This report presents a case study of a software project using a design-based research approach and Free/Open Source Software (FOSS). The project is developing a 3D virtual learning environment (3D VLE) for youth with autism spectrum disorders (ASD) to develop social competence. The potential of FOSS when implemented within a DBR framework is highlighted using examples from our process of designing and developing the 3D VLE, and the design principles resulting from our processes for fostering social interaction in the medium are presented. Using findings from five studies performed over the course of our DBR process, this article illustrates how the flexibility and community features of FOSS fit with the iterative nature of design-based research to benefit the development of our project and to forward understanding of how to support social interaction in the 3D VLE for individuals with ASD. Limitations of the approach are discussed.