The Open DOI resolver searches multiple repositories.
CORE Searches institutional repositories, subject-repositories and journal publishers worldwide. Very nice interface.
OpenDOAR Searches content in repositories worldwide.
OpenAIRE Searches content held in European repositories.
PubMed Central(PMC) a free archive of biomedical and life sciences journal literature, from the National Library of Medicine.
Europe PMC Great for searching the biomedical literature. Very user friendly interface. Includes U.S. Pubmed Central records in addition to European works.
OAIster A collaboration between the University of Michigan and OCLC. A union catalog of millions of records representing open access resources that was built by harvesting from open access collections worldwide using the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH).
Digital Commons Network Search across repositories hosted on this Bepress platform. Includes CUNY's Academic Works.