Mendeley is a freely available reference manager and academic social network designed to hep organise documents, collaborate with others online, and discover the latest research. it allows you to make your own searchable library, cite as you write, and read and annotate PDFs.
Mendeley has web-based, desktop and mobile versions. It requires you to store all basic citation data on its servers. Upon registration, Mendeley provides you with 2 GB of free web storage space, which is upgradeable at a cost.
Warning: Mendeley is hosted on servers in the U.S.