Zotero: pronounced [zoh-TAIR-oh]. Albanian word for "to acquire, to master"
Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share your research sources. It lives right where you do your work—in the web browser itself.
Mendeley is a free online service that allows you to index and organize your PDF documents, collaborate with fellow researchers and share information via shared and public collections, and discover new research through the Mendeley research network.
You can create reference lists and bibliographies in Word (Windows only) or OpenOffice (all platforms).
Interesting fact: Mendeley was founded in November 2007 by three German PhD students based in London. When Elsevier purchased Mendeley in 2013, it created a fuss on scientific networks and media. It also opened up the scope for Open Access (the huge collaboration features)