Zotero

Zotero is a free, open source, reference manager that works as a Firefox extension (making it cross-platform).  Each version is better than the last, and while I don’t use it exclusively, I find it incredibly helpful while browsing for articles.  It makes it incredibly easy to grab all of the citations off of a web-page such as a google scholar listing, and boasts many of the same features as Endnote including pdf storage and Word integration.  It’s not yet my primary reference manager, but it may be soon!

Incidentally, if you are already using Zotero, you can use Zotero to grab citations for any articles that I have a PDF for simply by viewing the page on which they are listed.