CORE - COnnecting REpositories ( Knowledge Media institute, The Open University) - millions of scholarly articles aggregated from many Open Access repositories.
Europeana - Multi-lingual online collection of millions of digitized items from European museums, libraries, archives and multi-media collections.
NIST Data Gateway - Access to all databases available from NIST's Standard Reference Data (SRD) collections.
OAIster - collection of digital open access/open archive documents (OCLC)
Science.gov - Gateway to authoritative selected science information provided by U.S. Government agencies, including research and development results
U.S. Dept. of Energy OSTI - Now the primary search engine for all DOE publications & resources. Replaces the National Library of Energy, SciTech Connect, PubScience, Energy Citation Database, & many other search interfaces. Maintained by the Office of Scientific and Technical Information.
WorldWideScience.org - Open access portal & federated search engine covering over 400 million pages from 65 countries. Papers can be translated into nine different languages.