PubMed (U.S. National Library of Medicine): Search biomedical literature
ACS Publications : American Chemical Society journals, books, etc. Covers 1879 to now
Science Direct : Search for articles published by the publisher Elsevier.
SciFinder : Search by topic, chemical, or reaction. You need to register before using this. See SciFinder tab in this guide for more information
Web of Science : A must for a thorough literature review, and accurate citation information. Allows for you to drill down into articles that have cited a publication
BioOne : Articles in the biological, ecological, and environmental sciences
Medline : Articles in medical, nursing and dentistry journals