Games:
Habbo : A virtual world game (like Second Life or The Sims) set in a hotel where you can hang out and chat with other users.
Uberstrike : Play virtual paintball solo or with a group.
Runescape : A fantasy adventure game
Pen-paper : A database of role playing games
Webgame14 : Another website offering a variety of games. Find out what is popular with other users,
TeenSay : A variety of games categorized by type includes chess, sports, fishing, etc.

Other fun stuff:
Truth or Fiction : heard a rumor? Check here to see if it is truth or fiction
Snopes : inquiry into urban legends, common fallacies, misinformation, old wives' tales, strange news stories, rumors, celebrity gossip, and similar items.
Mythbusters : a website for the Discovery Channel show Mythbusters includes quizzes, games, videos and results of previous inquiry.
Funtrivia : Like the title says: Fun trivia. Also, game and quizzes.

Magazines and Zines online:
Rookie Mag : A teen magazine for teenage girls, by teenage girls, updated daily with advice, music, books and style.
The Onion : A farcical, funny newspaper full of articles which highlight social commentary
The AV Club : From the people who brought you The Onion--a website focusing on arts and culture, featuring television, video and book reviews
The Source : Hip-hop music, culture and politics
Twist Magazine : Style and fashion for the modern teen girl
Thrasher Magazine : All skate culture, all the time
1UP : An online gaming magazine, featuring reviews and cheat codes
Bamboozled! : A zine by teens, for teens featuring movie and music reviews, articles and online forums

Webcomics:
Sharing Machine Comics : A hub for the webcomics Married to the Sea, Toothpaste for Dinner and Natlie Dee
Fluff in Brooklyn : A selection of webcomics and podcasts
IPL Graphic Novels : Nuts and bolts guide to comics and graphic novels, their history and importance, plus a great collection of links