In dynamic distributed computing environments, system entities may be classified into two main categories that are, in principle, in conflict. These are the Service Resource Requestors (SRRs) wishing to use services and/or exploit resources offered by the other system entities and the Service Resource Providers (SRPs) that offer the services/resources requested. Seeking for the maximisation of their welfare, while achieving their own goals and aims, entities may misbehave (intentionally or unintentionally), thus, leading to a significant deterioration of system's performance. In this study, a reputation mechanism is proposed which helps estimating SRPs trustworthiness and predicting their future behaviour, taking into account their past performance in consistently satisfying SRRs' expectations. The reputation mechanism is distributed, considers both first-hand information (acquired from the SRR's direct past experiences with the SRPs) and second-hand information (disseminated from other SRRs' past experiences with the SRPs), while it exhibits a robust behaviour against inaccurate reputation ratings.