MathSciNet
A database of reviews, abstracts and bibliographic information for much of the mathematical sciences literature published since 1940. Search options include author, title, keyword, and MSC (Mathematical Subject Classifcation).
Citation databases allow you to see how often a paper has been cited and follow connections between related works.
Web of Science
Interdisciplinary coverage of major science publications from 1900 to present.
Scopus
Scopus is multidisciplinary database that covers science, technology, and medicine. Cited references are included from 1970.
Google Scholar
Freely available resource.