Zotero is a free, open-source, reference management software that helps users collect, organize, cite, and manage their resources. The web-browser plugin keeps Zotero easily accessible while you work on research.
Zotero lets you attach PDFs, notes, and images to your citations, making annotation easy. You can also create folders and collections to organize your resources by project, as well as create bibliographies.
While you collect citations in Zotero, you also build a personal online database for your research. Most references can be imported with the click of a button from research databases and websites.
Zotero can also help format your references in Microsoft Word (or other word processors) in hundreds of citation styles.
Sharing and collaborating is possible through Zotero Groups.