This paper presents a concept for an e-Science oriented opportunistic cloud computing service, as an infrastructure to offer to researchers computing resources with the flexibility and ease of use of a cloud computing service, supported through the opportunistic use of the idle computing resources available in academic institutions. Also we present the work done in the context of the UnaCloud project, a first implementation of this concept, the experiences with the use of the tool to support real research projects, the existing perspectives for current and future work on this project, related work about different concepts of opportunistic or volunteer cloud computing, and the significance and potential impact of this project in the regional research community.