RefWorks is a tool for organizing and sharing your research materials and citing them in a wide range of citation styles.
Zotero is an easy to use application for organizing your research materials, sharing them with collaborators, and citing them in a wide range of citation styles. It's free. Works best with Firefox.
Refworks: Annotate pdfs, cloud based.
Zotero: cloud and web based
Both: organize sources, sharing folders, importing citations from databases, capturing citations from websites, create citations.
The Writing Center at Purdue University has created the most widely used, comprehensive guides to citation available.