This is a really interesting article about the story of one of the most popular web-based games of last year. The interesting part is that the creator of the game designed it as a joke, trying to poke fun at other wildly popular games like Farmville. He made the game as simple as possible, compelled users to log in regularly to "Click a Cow," and had them earn points when other people clicked their cows. The message was supposed to be, "these types of games are manipulative in their tactics to keep people playing, and they are mindless and painfully stupid." However, his design stumbled on something that people really enjoy, and as a result, people didn't get that the game was meant to be a joke, and it became a huge hit. I think this shows that flow-inducing actitivities, or ones that keep the user engaged and provide quick feedback, are highly desirable for users.