Zotero is a free easy-to-use tool that helps you collect, organize, cite, and share research articles. Zotero is available for Windows, Mac, and Linux.
Zotero allows the collection of citations to any kind of material and automatically formats bibliographies in almost any style. Zotero also has many search, tagging, and note taking features.
Note that Zotero supplies 300 MB of free storage for PDFs, images, web snapshots, and any other Zotero files. Individual subscriptions for additional storage space are available.
