"Scopus is the world's largest abstracts database of 13,450 titles from more than 4,000 international publishers with access to over 25 million abstracts (going back to 1966)." Full-text is included for some citations.
PubMed is a project developed by the National Center for Biotechnology Information (NCBI) at the National Library of Medicine (NLM), located at the National Institutes of Health (NIH)--Overview. NLM's search service to access the 9 million citations in MEDLINE and Pre-MEDLINE (with links to participating on-line journals), and other related databases --Home page.
Knovel provides a one-stop e-book source for finding answers to science and engineering questions. Subject areas include chemistry and plastics, materials science, environmental science, microelectronics materials, electrical engineering, electronic engineering, chemical engineering, systems engineering, food sciences, and mechanical engineering.