In collaborative systems, a set of organizations shares their computing resources, such as compute cycles, storage space or on-line services, in order to establish Virtual Organizations (VOs) aimed at achieving common tasks. The formation and operation of Virtual Organizations involve establishing trust among their members and reputation is one measure by which such trust can be quantified and reasoned about. In this paper, we contribute to research in the area of trust for collaborative computing systems along two directions: first, we provide a survey on the main reputation-based systems that fulfil the trust requirements for collaborative systems, including reputation systems designed for e-commerce, agent-based environments, and Peer-to-Peer computing and Grid-based systems. Second, we present a model for reputation management for Grid Virtual Organizations that is based on utility computing and that can be used to rate users according to their resource usage and resources and their providers according to the quality of service they deliver. We also demonstrate, through Grid simulations, how the model can be used in improving completion and welfare in Virtual Organizations. Copyright © 2009 John Wiley & Sons, Ltd.