Mendeley is a free reference management tool that allows you to keep track of your sources when doing research or writing a paper. Create your own personal library and annotate the saved PDF files. Insert in-text citations directly into your document and create bibliographies based on the references used.
You can access your Mendeley library irrespective of platform.
Zotero is a free, open source reference manager tool for both MAC and PC. You can use Zotero to keep track of your references (books, articles, websites) and drag them into your document as in-text citations. With this program you can get help with making in-text citations and generate a bibliography in a quick way.