http://www.popcap.com/games/pvzMan this is actually really addicting. The basic premise is that zombies are attacking your house, and you have to grow plants to fight them off. It's basically a tower defense, except the enemy moves down a horizontal lane instead of a winding path, and they come into direct contact with your towers (as opposed to more standard TD games where your towers are off to the side and invincible. It seems really simple at first but it's incredibly addicting because of all the strategy that becomes available. Your basic resource is "Sun", which you use to grow something like 40 different plants. Sun is gathered during daylight levels by clicking it as it randomly falls down the screen. There are also Sunflowers you can grow early on which produce it, and are required on the nighttime levels. You have other basic plants like the Peashooter which damages zombies, and the Wallnut which blocks them and forces them to attack it. Like I said before, there are like 40 different plants that you collect, but you can only carry 7-9 into a level.
That's another of the addicting aspects, gathering plants and trying out new strategies. The game is well balanced, and every plant is useful in different situations. There are a ton of different zombies types and different plants are effective against different ones.
I picked it up 3 or 4 days ago, and I haven't really touched anything else. I'm still not through the Adventure mode, and there's a ton of minigames and shit to unlock too. It's a pretty decent way to spend 10 bucks.
TALK ABOUT PLANTS VS ZOMBIES!
Oh, and get it on Steam, there's achievements if you care about those plus it's 10 bucks cheaper than off PopCap's own website.
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Plants vs Zombies!!!