With Connotea closing, Mendeley gobbled up by Elsevier, it seems that Zotero may be the last, best, free (as in freedom) reference manager.  Many people have noted that Mendeley’s business model may well have been to be bought up by a larger company.  In contrast, because of the way Zotero is run as a non-profit, it is likely to survive.  It’s also free (as in no money)!