An open access repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, physics, astronomy, electrical engineering, computer science, etc. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository.
The SAO/NASA Astrophysics Data System
Contains over 12.8 million records in astronomy and astrophysics, physics, and the arXiv-e-prints. Abstracts and full-text of major astronomy and physics publications are indexed. Also provides links to astronomical object information, data catalogs and data sets hosted by external archives.
NTRS: NASA Technical Reports Server
Provides access to NASA's technical literature, literature from the National Advisory Committee for Aeronautics (NACA), the NASA Image Exchange, etc.
The primary search tool for Department of Energy (DOE) funded science, technology, and engineering research and development (R&D) results and the organizational hub for information about the DOE Office of Scientific and Technical Information (OSTI).
Physics - A Guide to Web Resources
Created by Michael Knee of University at Albany, State University of New York, which is an excellent guide to the best the web has to offer for physics.
Alsos Digital Library for Nuclear Issues
Site closed August 19, 2019 due to security issues. They are working to make the content available sometime in the future.
An annotated bibliography of over 3,000 books, articles, films, CDs, and websites about a broad range of nuclear issues.