Zotero (pronounced "zoh-TAIR-oh") is a user friendly, free, open-source program that collects, manages, and cites research sources.
Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects, and create bibliographies.
It automatically updates itself periodically to work with new online sources and new bibliographic styles
Jason Puckett over at GSU created an excellent guide on using Zotero. In fact, it's so excellent that Zotero links to it in their "adopt" documentation.