Questimate! from Motion Math is a enjoyable estimation game in which you create off-the-wall questions and try to estimate their answers. It's a challenge you can play alone or against others in either pass-n-play mode or Gamecenter mode.
As its name implies, Questimate! is based on quests that are comprised of a series of questions. You are given three lives, which allows you to make three mistakes as you progress along a quest. It's a game of estimates so you don't have to be perfect, but you do have to answer most of the questions in a quest reasonably well if you want to win. The app includes a free intro quest, with additional quests
