Scopus offers broad coverage of scientific, technical, medical and social science literature. It includes academic, peer-reviewed and trade journals, as well as patents and scientifically oriented websites. Also covers all Medline content.
PubMed comprises more than 20 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites. This is the freely available interface to the largest database of biomedical information in the world.
JSTOR covers complete back issues of journals of major research journals in a variety of academic disciplines. (Coverage ends at 4 years prior to the present year.) This is a great resource for back issues of journals like Science.
GreenFile focuses on humans' impact on the environment. Its collection of titles includes content on global warming, green building, pollution, sustainable agriculture, renewable energy, recycling, and more.