Zotero

Zotero is:

a research tool that helps users collect and organize sources.
a free, open-source program that can be downloaded as a browser extension for Firefox, Chrome, and Safari and as a standalone program that works with Windows, Mac, or Linux systems.
a desktop application that lives in your browser.

Zotero's basic File Storage plan (300mb of storage) is free, but there is a charge for more storage:

2GB for $20/year; 6GB for $60/year; 10 GB for $100/year; 25GB for $240/year; Bigger plans available

(Source: Zotero File Storage)

Zotero does not charge for collaboration between users, and allows for institutional storage plans.

Firefox Extension
Download the Firefox plug-in.
Zotero Standalone for Windows
Zotero Standalone for Mac OS X
Zotero Standalone for Linux i686
Zotero Standalone for Linux x86_64
Zotero Connector with Chrome
Word Processor Plugins
You can integrate Zotero into your word processor (Word and OpenOffice) so you can use it as a citation manager in-text as well as for your bibliography. There is a handy video showing how it works with Word.

Quick Tour Video

See all Zotero tutorials here.

Using Zotero with LaTeX

A research guide from MIT discusses how to use Zotero in conjunction with the typesetting software LaTeX and its accompanying bibliographic program, BibTeX.