Zotero is a free citation management software available for Windows, Mac, and Linux systems. It is open source, and allows users to collect, manage, and cite their sources for any kind of project.
Zotero also has a PDF viewer and a group library feature within the program.
A big thanks to UIUC's guide on Zotero, which heavily informed this guide.
Want to know even more on Zotero? Head to the Zotero Citation Management Tool guide at SU.