Google Books
Google Scholar
Scinapse
Scopus
SemanticScholar
Biohunter
Mendeley
Zotero
Qiqqa
Mendeley
Zotero
JabRef
Citavi
Docear
Paperpile
BibDesk
Google BooksQiqqa is by far the strongest solution for research and managing pdf's. Zotero, named as a comparison is weak as it does not allow pdf;s to be loaded into its interface - you need to use a pdf reader, which defeats its whole purpose. With Qiqqa documents in a library can be searched together. It has excellent maps of your libraries. It is not prefect, having a few glitches, but still is the only one of its kind.
Google Scholar - Google Scholar is a freely accessible web search engine that indexes the full text of scholarly...
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Scinapse - Scinapse is a free, nonprofit, Academic search engine for papers, serviced by Pluto Network.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Scopus - Scopus is a bibliographic database containing abstracts and citations for academic journal articles.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .
