This report provides an overview of a diverse set of more than thirty digital library aggregation services, organizes them into functional clusters, and then evaluates them more fully from the perspective of an informed user. Most of the services under review rely wholly or partially on the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH), although some of them predate its inception and a few use predominantly Z39.50 protocols. In the opening section of this report, each service is annotated with its organizational affiliation, subject coverage, function, audience, status, and size. Critical issues surrounding each of these elements are presented in order to provide the reader with an appreciation of the nuances inherent in seemingly straightforward factual information, such as "audience" or "size." Each service is then grouped into one of five functional clusters: • open access e-print archives and servers; • cross-archive search services and aggregators; • from digital collections to digital library environments; • from peer-reviewed "referratories" to portal services; • specialized search engines.