Plants Vs. Zombies is a Popcap game where you defend a garden from a zombie apocalypse by using plants. This sounds a bit insane, but it's actually pretty fun. You unlock various plants that defend your garden in various ways, such as being literal peashooters, just acting as a wall, and catapulting watermelons. This game was announced on April Fool's Day of.. I think it was 2009, but I could be wrong, and everyone thought it was a joke. Blam. PopCap suddenly said "Oh, hey, it's real. Have fun." The game then went on to garner great reviews and cause people to have fun. Here's the trailer they launched, for reference: http://www.youtube.com/watch?v=CHAbHz8iYHc
The graphics of this game are cheerful, there are some very funny lines, the general plot is (to be a bit Firefly-esque) just crazy enough to work, and the game is now on sale for 3$ on Steam. Get it; the sale won't last long. The only con I have with this game is that after you beat it, there's not much replay value, but it's 3$- what can you lose?