Current cloud services are deployed on wellprovisioned and centrally controlled infrastructures. However, there are several classes of services for which the current cloud model may not fit well: some do not need strong performance guarantees, the pricing may be too expensive for some, and some may be constrained by the data movement costs to the cloud. To satisfy the requirements of such services, we propose the idea of using distributed voluntary resources---those donated by end-user hosts---to form nebulas: more dispersed, lessmanaged clouds. We first discuss the requirements of cloud services and the challenges in meeting these requirements in such voluntary clouds. We then present some possible solutions to these challenges and also discuss opportunities for further improvements to make nebulas a viable cloud paradigm.