Mendeley is a reference manager and academic social network with a free version that can help you organize your research, collaborate with others online, and discover the latest research.
from Mendeley website: www.mendeley.com
Great for managing PDFs
Has a social aspect. Can see what other users are reading and citing. Find other members with common research interests.
Does an excellent job of pulling citation metadata from PDFs
Can share citations and documents with others (*this function is very limited in free version*)