You can't have Halloween without subversive social commentary... and zombies!
I'm not sure how I managed not to see the original Dawn of the Dead before now, but I thought it was pretty great. Sure the acting is a bit cheesy and the special effects aren't great, but its a movie about a zombie plague overtaking the world. The social commentary on consumerism and greed are probably more relevant now than when the film was made. I've seen some of the later sequels and Night of the Living Dead is available through archive.org but I think I'm going to have to check out some other George Remero.
Oh and 28 Days Later is now available for free via hulu. I think you need to sign in to see it because its rated R, but its worth checking out.