The world’s largest search engine brings you the world’s largest academic search engine. It has pretty good coverage, and you can directly copy the citation for a result. Good place to start searching for papers.
Unpaywall is a Chrome extension that shows you a little green lock when there’s a legal open-access of a paper you’re trying to find available for download.
Pronounced as “archive” because the X represents the Greek letter chi. It is an open access website for uploading e-prints of papers. The papers aren’t necessarily peer reviewed, but a lot of the published papers in the core sciences end up here.
Some other sources
http://academic.live.com - Live Search Academic
http://www.science.gov/ - USA government for Science
http://citeseerx.ist.psu.edu/index - Computing & Information Sciences
http://www.mendeley.com/ - academic social network/collaboration
http://www.worldcat.org/ - Multidisciplinary
http://libserver.cedefop.europa.eu - Voc & Tech Education
http://inspirehep.net/ - Stanford physics information retrieval system
www.ssrn.com/ - Social Science Research Network