Originally Posted by
stifle
NEX is not free on all JR rail passes; you may need a limited express supplement ticket.
According to this site, NEX seems to be free with the Japan Rail Pass:
By JR Narita Express
"Direct, 60 minutes, 1500-3140 yen, 1-2 connections per hour
The fastest way of travel between Narita Airport and Tokyo Station is the JR Narita Express (NEX). The one way journey takes roughly one hour, costs around 3000 yen and is fully covered by the Japan Rail Pass and JR East Pass. There are departures every 30 to 60 minutes. "
http://www.japan-guide.com/e/e2027.html