1968
This cult hit by George Romero is one of the most important films in american cinema. Although it is not the first zombie film, Night of the Living Dead is the progenitor of the contemporary "zombie apocalypse" sub-genre of horror film, and it influenced the modern pop-culture zombie archetype. The film has also effectively redefined the use of the term Zombie. Early zombie films — Victor Halperin's White Zombie (1932), Jacques Tourneur's I Walked with a Zombie (1943) — concerned living people enslaved by a Voodoo witch doctor; many were set in the Caribbean.
The film may be one of the most widely available of the public domain titles, especially in the older vhs format. Look at gas stations, discount stores, convenience stores swap meets, and flea markets. Also, the film is available online as a free download.