Mendeley is available for free to everyone.
Mendeley is a desktop-based reference management tool. Mendeley's collaborative features allow you to share what you are reading with your colleagues and network with academics around the world. Mendeley is most useful for:
PDF management
Automatically creates citation from PDFs stored on your computer
Search full text feature allows you to easily find PDFs within your Mendeley library
Annotate PDFs
Social networking
Create or join open or invite-only groups to share what you are reading and connect with colleagues
Construct an academic profile, upload your research articles and track who is reading your articles
Working from anywhere
Online, offline or on a mobile device