The best replacement for zotero is endnote.

The best alternatives to zotero are rated as following:
★★★★★  endnote
★★★★★  mendeley
★★★  evernote
★★★  papers
★★  refworks
★  bibdesk
★  sente
★  jabref
★  diigo
★  bookends
★  qiqqa
  citeulike
  endnote web
  colwiz
  readcube

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Speaking about zotero, one probably shouldn't forget about scrivener.

Other search terms related to zotero and their ratings are:
★★★★★ scrivener
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