Zotero Online
Import PDFs directly into your personal Zotero Library. Import website links into your personal Zotero Library. Create groups to share PDFs and/or citations. Create in-text and works cited lists. Save citations online or through the desktop download. Zotero's online component works like the cloud, syncing your library across computers and online. Zotero can’t psychically cite that for you, of course. It has to have be described online. And the degree to which you will get proper citation details depends on how well the “metadata” of the source has been input online.
Zotero Online Reference
A nice overview from the folks at Zotero, making their case for Why Zotero?
What's with this guide?
Zotero's own website will tell you most of what you need to know to install and use Zotero. I created this library guide to supplement what it provides (you can also find some excellent tutorials and other how-to resources from other libraries on the Using Zotero... tab).
Zotero Citation
Updates:
Zotero Bib
12/22/17: Most of this guide has been revised for Zotero's major 5.x update except, I think, the Setting up Zotero and Zotero toolbar stuff (look in the DIY tab for a screencast on how to do this now).
Zotero Pdf
1/25/19 - I keep forgetting to add that Zotero now works with Google Docs (beyond the old drag-and-drop) See Using Zotero with Google Docs. As of July 2019, you can also move citations between Word and Docs.
1/25/19 - Also, Zotero is now integrated with Unpaywall (a free service that harvests open access articles from publishers and repositories - it has a nice browser plugin that I recommend installing), so that if your article is behind a paywall you don't have access to, it will search for an OA version.
6/17/19 - Zotero is now also integrated with the amazing Retraction Watch and will now automatically check your database and documents for works that have been retracted.