Plague Inc.

The title may sound grim, but who knew that decimating the world with some horrible disease would be so fun -- and addicting?

Plague Inc. is one of two hit strategy games in the App Store that challenge you to wipe out the human race (Pandemic 2.5, a port of the Flash game, is the other), and there are a few different ways to go about achieving your objective, though it's harder than it seems. Do you try to spread your disease rapidly but hold back its potency so it can fly under the radar? Or do you spend your points on making it more lethal and kill faster, thus slowing a cure, but potentially wiping out your hosts before it infects everyone? Ah, the choices.

Price: 99 cents (universal app)