Drop what you're doing and buy this game... right now.
The game consists of you defending your house against waves of zombies, using plants. Each plant has a different ability, like shooting peas at the zombies, generating sunlight (which is the monetary unit used to buy more plants), eating one zombie, etc. But you are limited to the number of plants that you can use at one time, which enhances the strategy portion of the game.
Each person will come up with his or her own tactics; where to place the different plants, what order to place them in, etc.
You can tell Popcap gave this game extra love, because the developers thought of everything. Just when a wave of zombies are approaching your house, and they've eaten through every plant you have, you get one last chance to take them out; a lawnmower will save you but once and take out those few that managed to slip through your defenses.
The music is fun and quirky, and the sound effects of the zombies match perfectly.
Seriously, go buy this game.