arXiv.org
Open access to e-prints in Physics, Mathematics, Computer Science, Quantitative Biology and Statistics.

CiteSeer: Scientific Literature Digital Library
A scientific literature digital library and search engine that focuses primarily on the literature in computer and information science hosted at Penn State's School of Information Sciences and Technology.

Digital Library Network for Engineering and Technology
A well categorized repository of "Learning Objects" such as reports, presentations and manuals. An initiative of the National Science Foundation.

Directory of Open Access Journals
A listing of fully open access (free, full text), peer-reviewed scholarly journals covering all subjects and languages.

efunda: engineering fundamentals
Concise and reliable reference information for engineers categorized into six sections: Materials, Design, Processes, Units, Formulas, and Math.

E-print Network
A vast, integrated network of electronic scientific and technical information communicating the results of research activities primarily in physics, but also in chemistry, biology, materials science, nuclear sciences and engineering, energy research, computer and information technologies, and other disciplines. A product of the Office of Scientific & Technical Information.

FreeScience.info
Over a thousand scientific books free to download.

Infomine
Physical Sciences, Engineering, Computing and Math searchable database of Internet resources such as databases, electronic journals, electronic books, bulletin boards, mailing lists, online library card catalogs, articles, directories of researchers, and many other types of information.

Intute: Science Engineering and Technology
An online service providing access to the quality web resources for education and research in science & technology. Includes a set of free online tutorials designed to help students improve their Internet information literacy and IT skills. Created by a network of UK universities and partners.

Library of Congress - Science Reference Services
A host of science subject guides and bibliographies. Also includes a wide range of webcasts of visiting scientists.

Martindale's The Reference Desk
Remarkable set of categorized web reference resources. Categories include: cyberinfrastructure, electronic media, chemistry, engineering, materials science, health science, and many more.

National Academies Press (NAP)
The NAP publishes more than 200 books a year on a wide range of topics in science, engineering, and health. Thousands of books can be downloaded for free, while others are available for purchase.

National Science Digital Library (NSDL)
Established by the National Science Foundation in 2000 as “the Nation’s online library for education and research in science, technology, engineering, and mathematics.”

National Technical Information Service (NTIS)
Serves our nation as the largest central resource for government-funded scientific, technical, engineering, and business related information available today. Here you will find information covering over 350 subject areas from over 200 federal agencies.

SciCentral
A gateway to scientific research news resources. Includes links to sites in the fields of Biosciences, Health Sciences, Physics/Chemistry, Earth & Space, and Engineering.

Science.gov
A gateway to authoritative selected science information provided by U.S. Government agencies including research and development results.

Scirus
An internet search engine which only deals in quality scientific and technical information, and searches over 200 million web pages. It has basic and advanced search options, and uses an algorithm to correctly rank search results by relevancy.

SciTechResource.gov
An online catalog of government science and technology web sites managed by the U.S Department of Commence.

TechXtra
A search service which can help you find articles, books, websites, industry news, job announcements, technical reports, technical data, full text eprints, research, thesis & dissertations, teaching and learning resources and more, in engineering, mathematics and computing.

Yahoo Science Directory
A large set of links in a variety of fields of science.

WorldWideScience
Allows you to search multiple national and international scientific databases at once.  Developed and maintained by the Office of Scientific and Technical Information