PubMed
PubMed comprises more than 20 million citations for biomedical
literature, life science journals, and online books. Citations may
include links to full-text content
Google
Scholar
Provides a search of scholarly literature across many disciplines
including theses, books, abstracts, patents and articles
PubMed
Central
A free digital archive of biomedical and life sciences journal
literature
Scirus
A comprehensive scientific research tool allows search journals,
scientists' homepages, courseware, pre-print server material, patents
etc.
Scopus
A large abstract and citation database of research literature and web
sources