The need to use resource discovery and composition tools to assist in the development of scientific workflows is well established. While systems have been developed to guide scientists in the design and implementation of their protocols into executable workflows, these systems differ significantly in the way they support the various steps of resource discovery. This paper proposes a classification of resource exploration and discovery tools according to five main categories: content, graphical interface, maintenance, optimization, and query. The paper overviews six resource discovery approaches and evaluates them according to the proposed classification. An example of how the metrics can be used in the selection of an appropriate tool for given requirements is presented.