Pontypool - Or rather, Noam Chomsky's Dawn of the Dead. Just when you think there's nothing left to be done with the Zombie Movie genre, a gem like this one comes along and surprises you. Seriously, the vector for infection is unlike anything I've seen in movies or literature, and actually kept me interested. The setting is claustrophobic, the acting is superb, and it's developed in such a manner that keeps you involved until the bitter end.
The DVD is for sale on the movie's website, by the way.