One sure-fire way to make a good or great game better is to add a Zombie mode. This would probably also apply to bad games, where adding a Zombie mode would make them at least alright. With that idea in mind PWN or DIE thought of 8 new and old games that would benefit by adding zombies to them.
(Braid, Call of Duty: World at War, Halo 3, Madden NFL 09, Madden NFL 09 All-Play, Madden NFL 09: 20th Anniversary Collector's Edition, Madden NFL 2010, Mario Kart 64, Mario Kart Wii, PC, Portal, Portal 2, Portal: Prelude, Portal: Still Alive , PS3, Sim City 2000, Sim City 4 Deluxe, SimCity, SimCity 4, SimCity 5, Wii, Xbox 360)