Mendeley Reference Manager from Elsevier is a free web and desktop reference management application. It helps you simplify your reference management workflow. With it, you can store, organize, and search all your references from just one library; seamlessly insert references and bibliographies into Microsoft Word documents using Mendeley Cite; read, highlight and annotate PDFs and keep all your thoughts across multiple documents in one place; collaborate with others by sharing references and ideas.
Free for Mendeley Reference Manager (allows 2GB personal library space)
Premium:
Collaboration: Mendeley is also an academic social network that enables you to share your research with others. Researchers can collaborate online in public or private groups, and search for papers in the Mendeley group database of over 30 million papers. Mendeley helps you connect with other scholars and the latest research in your subject area.
Good for technical or scientific fields
Browser plugin for Firefox and Chrome--just bookmark a web page and it saves to your Mendeley library
Portability across different devices
Drag and drop functionality
Download, read, highlight, and annotate PDFs
Cannot generate standalone bibliographies. You either have to create one while writing a Word document, or copy and paste the references you want from Mendeley desktop into a Word document.
Monthly cost for large projects and teams
No bulk downloading of PDFs
Potential privacy issues since Mendeley is owned by Elsevier, a for-profit company that tracks users' research habits