This site - NZBSource.com - NZB Source - Updated NZB Links - has over 50 NZB sites listed. Not all are working but a good majority are. It tells which are free, which need a sign-up and which are search engine/index sites. It hasnt been update since 2007 though.
This site - http://www.indeklit.nl/ - has around 25 NZB sites listes (down the left hand side) and was updated this year.