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

ProCite - ProCite is a tool for creating citations according to the standard defined by the users.

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

Pybliographer - Pybliographer is a tool that helps you manage your bibliographic databases in a user-friendly way.

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.

I Librarian - I Librarian is an online tool that comes with a great way to handle and manage your PDF paper and office documents.