Zotero is a citation management tool that combines a free desktop application, browser extension, and word processing plug-ins to help you organize and cite your research. It provides 300 MB of free attachment space, plus options to upgrade storage starting at $20 per year. Zotero's word processor plug-ins allow you to insert citations as you write, and work with Microsoft Office, Libre Office, and Google Docs. You can register for Zotero here and download the Zotero desktop application here.
You can add references to your Zotero library in a number of different ways. Below are the most common ways; check out Zotero's "getting stuff in your library" guide for the full rundown.
USING THE BROWSER EXTENSION
DRAG & DROP PDF
USING AN IDENTIFIER
EXPORTING FROM @LL SEARCH