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

DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.

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

Keep It - Write notes, keep things, and find them again

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.

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