AutoZotBib is an extension for Zotero which will automatically export your Zotero library to a BibTeX file every time the library is changed.

More information, and documentation, is available at www.rtwilson.com/academic/autozotbib

Updates:

• v0.6 - Added option to turn off automatic exporting and click a menu item to export the BibTeX file (thanks bjonnh!)
• v0.5 - First major release of AutoZotBib