Clusters of workstations are a promising platform for high-performance computing. Most of such clusters are built of dedicated workstations that cannot be used for other purposes. Even though these clusters offer good price/performance ratio, their costs are not negligible. On the other hand, there exist many idle workstations connected via computer networks. The idea of exploiting such idle resources is obvious. In this paper, we describe an architecture of clusters made of non-dedicated idle workstations. These clusters aim at providing a single-system-image Linux environment. The main innovative feature is that a cluster administration is separate from administration of individual workstations. We discuss issues of security, availability, performance, and administration of such clusters. We also describe a pilot implementation and give promising experimental results.