The form factor of mobile devices remains small while their computing power grows at an accelerated rate. Prior work has explored expanding the output space by leveraging free displays in the environment. However, existing solutions often do not scale. In this paper we discuss Open Project, an end-to-end framework that allows a user to "project" a native mobile application onto a display using a phone camera, leveraging interaction spaces ranging from a PC monitor to a public wall-sized display. Any display becomes projectable instantaneously by simply accessing the lightweight Open Project server via a web browser. By distributing computation load onto each projecting mobile device, our framework easily scales for hosting many projection sessions and devices simultaneously. Our performance experiments and user studies indicated that Open Project supported a variety of useful collaborative, sharing scenarios and performed reliably in diverse settings.