Is there a website where I can access the complete and most recently updated version of the United States NEC for free?
Yes, the NEC. Is free once adopted . search for free NEC and you will find it. This is only for the "code" I don't think they release the hand book that has a lot of information on why, this is the version most inspectors use , but once adopted the NEC is free on line. Just checked look up nfpa.org free access just need to sign in..
It is free to observe it on the web, but it will not allow you to copy text, print the pages or export them.