IOS, Android, Windows Phone Free
QuizUp is a multiplayer trivia game that allows you to challenge friends or strangers on your own special topic of interest, ranging from things like American history to Harry Potter. To make things more interesting, each player gets a time bar that slowly runs down, so you can't take too long to answer a question. And if you really are sure of your stuff, you score extra points for beating your opponent to the punch. There's also a social aspect to the app; it allows you to find people based on age, location, and interest, then strike up conversations with like-minded folks.