Leave it to Zynga to make a game where you're essentially watching plants grow into a compelling social experience. By combining creating a virtual farm with two kinds of currency, experience points and levels, unlocking achievements, and interacting with friends, Zynga has built a casual social game that is both non-violent, pastoral, and strangely compelling. It's quite a distance from the vampire, pirate, dungeon hack, and mob war games that enjoy perennial popularity.