1. The Barnes & Noble Nook and Amazon Kindle
Be careful with this one. If you have a valid Netflix membership, you have full access to all of the titles available for instant streaming right on your phone. This can be amazing if you need a way to pass the time, something to watch when you're on the bike at the gym, etc. Though, spending too much time watching movies and tv shows could detract from your grades, so use this one sparingly. What I find useful is to do about an hour's worth of homework and then watch an episode of whatever show I'm on. Just try to resist the temptation to watch a movie in class!
Evernote is essentially the ultimate notebook. Whenever an idea pops into your head, you can record it into Evernote through various means. If you see something interesting in a book or on the whiteboard, snap a picture of it. You can even record voice and audio notes if you hear a certain sound you want to remember or just like to hear your voice. Obviously, you can also make regular text notes. Evernote connects to all your devices and accounts, allowing you to synchronize your thoughts among various media.
That's all I've got for now. I'll probably post another list of the top 3 android games, or maybe a few more useful apps, with shorter descriptions! I'm an android user to heart, so I won't be covering Apple products. Sorry!
So what are your favorite apps?