Is a free program for citations management and bibliography generation designed to be competitive with Endnote and similar products. I've been using it for a couple of weeks. Zotero lives as a Firefox extension and it's best feature is the ease with which you can import citations from the web. If you are looking at a paper on JSTOR, for example, you can "one-click import" the citation. One-click import is also available from Amazon, Cite-Seer, ABI-Inform, the Library of Congress, many university library catalogs, Medline, Google books and many others.
Thus it's very easy to generate a citations list in Zotero by visiting a handful of large databases - this is especially easy for books and not too hard for recent articles but it's more difficult to find older articles in online databases. Zotero's interface is somewhat clunky so entering citations by hand is not as convenient as I would like. In addition to grabbing the citation, Zotero can grab entire PDFs so you can keep articles and citations in one database. Exporting of the citations in a variety of bibliographic format is clean and well done.
Zotero is only available as a Firefox extension (the developers take a perverse pride in this fact). The developers are at GMU, although I don't know the team at all. Zotero will import citations from another citations management program so switching is low cost. Worth checking out.