Arxiv:

A highly-automated electronic archive and distribution server for research articles.  Open access to 964,211 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.

BASE:  Bielefeld:

BASE is one of the world's most voluminous search engines especially for academic open access web resources.  BASE provides more than 60 million documents from more than 3,000 sources.

Cambridge Open:

Open Access Journals to APSIPA Transactions on Signal and Information Processing, Forum of Mathematics,  Forum of Mathematics,  Global Mental Health,  High Power Laser Science and Engineering  and   Journal of Nutritional Science.

CERN Document Server:

CERN offers full text preprints, articles, books, journals, and other documents of interest to persons working in particle physics and its related areas.

D-Lib Magazine:

Is an electronic publication with a focus on digital library research and development, including new technologies, applications, and contextual social and economic issues. D-Lib Magazine appeals to a broad technical and professional audience.

Digital Commons Network:

full-text scholarly articles from hundreds of universities and colleges worldwide. The Digital Common Network includes a growing collection of peer-reviewed journal articles, book chapters, dissertations, working papers, conference proceedings, and other original scholarly work.

Directory of Open Access Journals (DOAJ):

The DOAJ aims to be comprehensive and cover all open access scientific and scholarly journals that use a quality control system to guarantee the content. In short, the DOAJ aims to be the one-stop shop for users of open access journals.

DMOZ Open Directory Project:

DMOZ is the largest, most comprehensive human-edited directory of the Web.

Elektronische Zeitschriftenbibliothek EZB (Electronic Journals Library):

Offers an effective use of both scientific and academic journals publishing full text articles. At the moment, it contains 77,555 titles, among them 14,296 online-only journals, covering all subjects. 46,536 journals can be read free-of-charge.

ERIC:

Provides access to educational literature and resources.

EUR-Lex:

Replacing the earlier service CELEX, EUR-Lex provides direct free access to European Union law. The system makes it possible to consult the Official Journal of the European Union and it includes inter alia the treaties, legislation, case-law and legislative proposals. It offers extensive search facilities.

Find Articles:

Provides free access to millions of articles from thousands of top publications.

HG.org Worldwide Legal Directories:

Was one of the very first online law and government sites, Founded in 1995 by Lex Mundi, a large network of independent law firms.

John Hopkins APL Technical Digest:

Johns Hopkins APL Technical Digest is published by the John Hopkins University Applied Physics Laboratory. The journal focuses on missiles, radar, sonar, senor, chemical biological and information technologies; space science and engineering, microelectronics; communications; navigations; modeling and simulation; advanced research and technology development.

Labordoc:

Contains references to a wide range of print and electronic publications, including journal articles from countries around the world, on all aspects of work and sustainable livelihoods, and the work related aspects of economic and social development and human rights.

Multidisciplinary Digital Publishing Institute:

Is a platform for peer-reviewed, scientific open-access journals operated by MDPI AG, based in Basel, Switzerland. MDPI publishes over 120 diverse open access electronic journals.

National Institute of Standards and Technology (NIST):

Allows access to NIST scientific and technical data, encompassing a wide range of information pertaining to varied scientific disciplines and links to some free online NIST database and articles published in the journal of Journal of Physical and Chemical Reference Data (JPCRD).

Scientific Journals International (SJI):

SJI publishes peer-reviewed open-access journals for all disciplines. SJI has assembled the most prestigious and extensive Editorial and Advisory Board in the world, representing scholars from Princeton, Yale, Oxford, Harvard, Cambridge, MIT, Columbia, and other leading universities from around the world.

Social Science Research Network:

SSRN eLibrary consists of two parts: an Abstract Database containing abstracts on over 557,500 scholarly working papers and forthcoming papers and an Electronic Paper Collection currently containing over 460,300 downloadable full text documents in Adobe Acrobat pdf format.

Springer Open:

Includes Springer’s portfolio of 160+ peer-reviewed fully open access journals across all areas of science ranging from very specialized titles to Springer Plus, our interdisciplinary open access journal that covers all disciplines.

Taylor & Francis Open and Routledge Open:

Publish a number of pure open access journals, with no subscription content. The articles in these journals receive both rigorous peer review and expedited online publication.

Wiley Open Access:

Is a program of fully open access journals.  All research articles published in Wiley Open Access journals are immediately freely available to read, download and share.

World Data Bank:

World Bank’s Open Data Initiative offers open and free access to over 2,000 financial, business, health, economic and human development data from more than 200 countries, with data going back 50 years and access to at least 20 World Bank Group Databases.