Update 4/20/17: Old article from 2015 but this website and app are still one of my favorites!
I am a self-proclaimed aviation geek and a couple of months ago I found an incredible app called Flightradar24. The app is based off of the website Flightradar24 which is extremely powerful and shows a live picture of the world map with almost every aircraft on it. You can select the aircraft and it will show where it came from, where it's going, and more detailed info. The best part is the phone app is just as powerful as the website.
There is a free version of the app available in both the iTunes and Android stores but it's a more neutered version of the website. Splurge just $3.99 and you can get the full featured app (Apple / Android), and I promise you won't regret it. I've seen quite a few aircraft recently flying overhead and wondered what they were. All I have to do is pull up the app and it shows the plane flying right overhead with very detailed info.
I know Mrs. TT is not very impressed with how much time I spend clicking on random planes on my phone, but I thoroughly enjoy it. There are also quite a few interesting aircraft such as the Airbus 380's, and Boeing 747/777/787's, which fly way up near the Arctic Circle. It's incredible to see and follow their flight paths.
Check out the Flightradar24 website and check out the Flightradar24 app on Apple or Android phones. It's awesome to follow a friend or family member's flight or just figure out what is flying over head. I use the app daily and I love to geek out with it. I think you will too. For only $3.99, it's about the cheapest entertainment you can find.