While not one of the high profile games, IUGO's Zombie Attack! [App Store, $1.99] has developed a bit of a following amongst our forum users. Although it generated a bit of a lukewarm initial response, the game has since proven itself to offer hours of zombie mangling fun for many (discussion thread). IUGO has now released a free Lite version [App Store] so you can try it yourself.
Zombie Attack is a non-traditional Tower Defense game where you control the movements of main character with either touch or accelerometer controls. To build towers in a particular spot, you need to actually run to that spot while avoiding the oncoming onslaught of Zombies. This features adds a bit of action to an otherwise strategy-only genre.
The Lite version limits your arsenal to one weapon and one 'shack bomb' but gives you the idea of the basic mechanics of gameplay. High scores and strategies have been discussed in our forum.