Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
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.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.
Thomson Reuters - The intelligence, technology and human expertise you need to find trusted answers. Thomson Reuters is The Answer Company.
Docear - Docear