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.
Both: organize sources, sharing folders, importing citations from databases, capturing citations from websites, create citations.
Zotero: cloud and web based