Mendeley is a combination of a desktop application and a website which helps you manage, share and discover both research content and research contacts. Mendeley also allows you to drag & drop PDFs into your Mendeley account and then highlight and annotate those PDFs and share them with others.
Mendeley's free version has some space limitation (2 GB of web storage, 500 MB each for personal storage), 3 users and 1 shared collection. Premium accounts are available.
NOTE: Mendeley is hosted on a US server and does not comply with BC's Freedom of Information and Protection of Privacy Act (FIPPA) which requires that the mandated use of systems located outside of Canada must be with prior written consent if the user has to provide personal information.