Zotero lives in your browser and on the web, at zotero.org.
Zotero can detect and import reference information from 1000's of databases and websites to create new references automatically. PDF's can even be automatically downloaded and attached to each reference - you'll never need to search for a PDF again! See Zotero Support - Getting Stuff Into Your Library for more.
Zotero integrates with word processing programs like Word & OpenOffice, with support for all the major citation styles and over 7000 more - you can see the full list, and download them all, here: Zotero Style Repository
Zotero allows sync across multiple computers and collaboration with groups of like minded researchers. including free online library storage for up to 300 MB
To download the Zotero Standalone & browser connectors