Mendeley is a free reference manager and academic social network.
Make your own fully-searchable library in seconds, cite as you write, and read and annotate your PDFs on any device on http://www.mendeley.com. You can:
Citation Plugin is a free, simple plugin for Microsoft Word (Windows, Mac) or Libre Office (All platforms).
Zotero is an easy-to-use tool that collects, manages, and cites research resources. It's easy to use, connects with your web browser to download sources, and best of all it's free.
Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects, and create bibliographies.
Zotero uses Citation Style Language (CSL) to properly format citations in many different bibliographic styles. Zotero supports all the major styles (Chicago, MLA, APA, Vancouver, etc.) as well as the specific styles for over 8,000 journals and publishers.
Zotero's Word and Libreffice plugins allow users to insert citations directly from their word processing software. This makes citing multiple pages, sources, or otherwise customizing citations a breeze. In-text citations, footnotes and endnotes are all supported. With community-developed plugins, Zotero can also be used with LaTeX, Scrivener, and numerous other writing programs.