Zotero-natbib is a project mainly written in JavaScript, it's free.
Zotero BibTeX translator for natbib format
This is a slight change to the standard Zotero BibTeX exporter in order to work better with natbib, and fix the letter "ø"... I am using it with these patched versions of [[http://github.com/unhammer/writer2latex][Writer2LaTeX]] and [[http://github.com/unhammer/lyz][LyZ]].
To use it: