"The SAO/NASA Astrophysics Data System (ADS) is a digital library portal for researchers in astronomy and physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant. The ADS maintains three bibliographic databases containing more than 13 million records covering publications in Astronomy and Astrophysics, Physics, and the arXiv e-prints."
"The American Physical Society represents some 48,000 physicists, and most of our work centers on scientific meetings and publications-the primary ways that physicists communicate with each other. With PhysicsCentral, we communicate the excitement and importance of physics to everyone."
"Science.gov searches over 60 databases and over 2,200 scientific websites to provide users with access to more than 200 million pages of authoritative federal science information including research and development results."