Plants vs. Zombies
One of the strangest and greatest tower defense games of all time, Plants vs. Zombies has finally shambled onto Android (albeit through the Amazon App Store). In PvZ, you must defend your lawn, pool, and roof from zombie hordes by means of various plants. Each plant has strengths and weaknesses, and you'll need to master all of them to survive each wave of the undead. The game is a pretty big download, so you should connect to a Wi-Fi network before making the transfer. Once you have it on your phone, though, those zombies won't know what's hit them.
Popcap's Plants vs. Zombies | Price: $2.99