Kill boredom

TypeRacer: Compete in a typing-speed game!
Khan Academy: Videos on anything you'd ever like to learn, and addictive Math problems (I've spent hours at a time here)
Memrise: Learn new, useful words in a fun way!
Sporcle: Speed Trivia (popular in college dorms)
Lumosity: The free games are fun and addictive themselves!
Lifehacker: Make the most out of your body, mind, and habits!
Wikipedia: Schools Wikipedia tailored to kids like you!
Wikipedia: Find something interesting to read, then explore!
Free Rice: Trivia game that donates food to needy families.
Google Books: Thousands of books to read, for free!
Grooveshark: Listen to music online & create playlists you can listen to later!
Kitten War: Which kitten is cuter???
Word Twist: Word Twist is like one-player Scrabble only faster!
Code Academy: Learn to create computer programs!

Learn a skill

Willpower trick - Help yourself resist temptation or focus more on tasks at hand!
Self Control - Yes, you can improve your basic self control!

Break a bad habit

Fight your bad instincts!
Hate your bad habits - I love the mantra "mind over matter" and I have great faith in this method :)
Don't wait to break that habit! - Happy statistics say many people break life-threatening habits, but why wait?

Create a good habit

Upgrade your daily routine - Great stuff here! Some is aimed more at working people, but I love this stuff!
Commitments work! - Study shows that real commitments help you stick to resolutions!
One good habit a month - Focus on just one big habit each month. Get one in before school starts!
Different strategies each day - Contrasting the one-a-month, try one-a-day to keep yourself fresh and find what really works for you!
Game addiction information - Understanding why you're addicted can help you cut back!