Science search engines, free

The Analytical Sciences Digital Library, ASDL, collects, catalogs, links and publishes peer reviewed web-based discovery materials pertinent to innovations in curricular development and supporting technical resources in the analytical sciences.

Open access to 909,799 e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.

BASE is a search engine especially for academic open access web resources.

BioOne is a global, not-for-profit collaboration bringing together scientific societies, publishers, and libraries to provide access to critical, peer-reviewed research in the biological, ecological, and environmental sciences.

The Information Bridge: DOE Scientific and Technical Information provides free public access to over 301,000 full-text documents and bibliographic citations of Department of Energy (DOE) research report literature. Documents are primarily from 1991 forward and were produced by DOE, the DOE contractor community, and/or DOE grantees. Legacy documents are added as they become available in electronic format.

ingentaconnect offers one of the most comprehensive collections of academic and professional research articles online - some 4.5 million articles from 13,500 publications.

The National Academy of Sciences (NAS) is a private, non-profit society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the public good.

The Grey Literature Network Service was founded in 1992. The goal of GreyNet is to facilitate dialog, research, and communication between persons and organisations in the field of grey literature. GreyNet further seeks to identify and distribute information on and about grey literature in networked environments. Its main activities include the International Conference Series on Grey Literature, the creation and maintenance of web-based resources, a moderated Listserv and combined Distribution List, The Grey Journal (TGJ), and curriculum development.

Grey Literature is a field in library and Information science that deals with the production, distribution, and access to multiple document types produced on all levels of government, academics, business, and organization in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body.

PLOS is a nonprofit, Open Access publisher empowering researchers to accelerate progress in science and medicine by leading a transformation in research communication.

PMC is a free full-text archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM).

Open Access e-journals.

Science.gov searches over 55 databases and over 2100 selected websites from 13 federal agencies, offering 200 million pages of authoritative U.S. government science information including research and development results. Science.gov is governed by the interagency Science.gov Alliance.

ScienceResearch.com is a free, publicly available deep web search engine that uses advanced "federated search technology" to return high quality results by submitting your search query - in real-time - to other well respected search engines then collating, ranking and dropping duplicates of the results.

Q-Sensei pools and processes a vast amount of information from the Internet and makes it better accessible to its users with its patented Search and Presentation engine. Search results are displayed clearly and comprehensively while cross references with rich content enable the user to gain more insight and understand search results in a larger context.

WorldWideScience.org is a global science gateway comprised of national and international scientific databases and portals.