Available via Steam or directly from Popcap, this is a pretty cute take on the classic tower defence games that anyone who's ever been bored at work will know of old. The basic gameplay doesn't break any new ground - zombies walk towards your house and you get to plant a variety of turrets to kill them before they make your back door and eat your brains. Over time you unlock new plants, encounter new zombie types and gain access to a bunch of puzzles and mini-games. There's also an item shop where you can buy upgrades of various types.
The game itself is fairly easy, you have to try pretty hard to fail a level in the core 'adventure' mode - at least the first time around. There's a Mario style boss fight at the end after which you go around again but with various twists applied to the levels.