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

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.

SemanticScholar - An academic search engine that utilizes artificial intelligence methods to provide highly relevant results and novel tools to filter them with ease.

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.

Scopus - Scopus is a bibliographic database containing abstracts and citations for academic journal articles.