Facebook - Social network
PayPal - To see how much money you got at the moment on your PayPal account, etc.
Google Chrome - Better browser than Safari.
Ebay - Order stuff through the app, they got nice stuff from all around the world.
Skype - If you use Skype, then download this aswell so you can be online through the phone and get messages while your computer is off.
SnapChat - Have som fun with your friends..
Instagram - Be Social, use Instagram
Google Maps - If you need to know how to get somewhere, what path to take, when the bus is coming, etc. Use Google Maps, it's very useful when you are going somewhere you've never been before.