Casual game maker Zynga made its name with FarmVille where you can create and tend your own virtual crops and livestock. A weird little game, primitive in a lot of ways, but it really caught on. You play it out of your browser, usually on Facebook, and it's free. But the sequel is way more popular than the original. Zynga released CityVille December 2 and already has nearly 72.5 million users. FarmVille: 57.4 million users. In CityVille, you don't fret over your imaginary cows and strawberry patches, you're a city planner laying down roads, putting up coffee shops and big buildings. You can let your friends run a business in your city. The thing about these games is that they compel you to come back and play them some more. You let things grow while you're away from the computer and then you come back to see the rewards. They also offer in-game purchases where you can acquire more and level up. Zynga is making an absolute killing here and redefining online gaming in the process.