The film is considered part of the public domain due to a mistake by the original distributor (Walter Reade Organisation) who neglected to place the proper copyright indications on the print. This allows the film to be legally & freely available to watch online or download through various torrent sites. If you want to download a copy of the film, the best place is the Internet Archive which has provided various versions in differing quality and video formats (http://archive.org/details/Night.Of.The.Living.Dead_1080p).
Synopsis: A group of survivors converge on a farmhouse, where they remain in a battle of survival against a zombie horde and each other as tensions mount and attitudes differ.
Night of the Living Dead