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 that allows you to highlight and annotate your sources. You can also create group libraries.
A big thanks to UIUC's guide on Zotero, which informed some of this guide.
If you installed the Zotero Connector for Firefox, Safari or Chrome, you are able to save citations automatically from within your browser to your Zotero library.
Multiple Citations
On some web pages that list information about multiple items (e.g. a list of database search results), Zotero will show a folder icon. Clicking this folder icon will open a window where you can select the items that you want to save to your library.
Adding PDFs
Add existing PDFs to your library by dragging them into the Zotero pane. Most of the time, Zotero will search for the metadata for that PDF and create an entry for it. If it does not, check your Settings or Preferences. In Edit > Settings > General, check the boxes for "Automatically retrieve metadata for PDFs and ebooks" and "Automatically rename locally added files."