Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...

Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.

Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.

PubMed.gov - PubMed comprises more than 29 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.

Qiqqa - Qiqqa is a free research and reference management software. It can be used in many organizational projects from the academic to the personal to the business endeavor. Read more about Qiqqa.

Scinapse - Scinapse is a free, nonprofit, Academic search engine for papers, serviced by Pluto Network.