An open archive of scientific papers including, but not limited to: Cosmology and Extragalactic Astrophysics; Earth and Planetary Astrophysics; Galaxy Astrophysics; High Energy Astrophysical Phenomena; Instrumentation and Methods for Astrophysics; and Solar and Stellar Astrophysics. In other words, more articles than you can shake a stick at.
Scopus is an abstract and citation database of peer-reviewed research literature, indexing over 19,000 titles from more than 5,000 international publishers. Scopus covers the scientific, technical, medical and social sciences fields as well as arts and humanities. Scopus includes scholarly journals, book series, conference proceedings, trade publications and Open Access journals.
Just like Google except this search engine crawls a corpus of scholarly articles. A good place to begin a search, especially a broad one. Result sets are typically large and unwieldy, making Scopus a better source for focused queries.
Applied Science & Technology Source offers extensive coverage of research and development within the applied sciences and computing disciplines by combining high-quality databases from both EBSCO Publishing and H.W. Wilson. This premier resource offers full text for almost 1,200 journals, citations to millions of articles. The content within Applied Science & Technology Source derives from numerous sources, including leading trade journals, professional and technical society journals, and conference proceedings.