Tumblr app is the epitome of not taking notes in class.
Instagram to take fancy pictures and to piss off hipsters.
Fruit Ninja 'cause it's fun.
World of Warcraft Armory (free) is so nifty when it comes to planning out gears and keeping updated on guild stuff.
Alfred is a pretty cool app that helps you figure out where you should go eat, based on your past experiences and likes/dislikes.
Words with Friends. Of course.