Not sure what you mean by "positive" here. Ethically, what you probably want are works that make you take the perspectives of people different from you, and involve solving problems in creative and effective ways. Maybe I'll try making a list of those later, but right now I'm just going to focus on fiction that might inspire people's problem-solving skills a little.
Top Recommendations:
Online original fiction: Everything written by Ted Chiang (excepting "Understand," perhaps - that is not a story about ethical behavior :P). "The Man Who Bridged The Mist" by Kij Johnson. Possibly some selected Greg Egan stories, possibly Yudkowsky's The Sword of Good.
Book-form fiction: Tons of options here. Rosemary Kirstein's "The Steerswoman's Road" is quite good. Asimov's novel "The Gods Themselves" is a classic (if PG-13), can also direct them to an Asimov anthology like "I, Robot." More classics like Lloyd Alexander's The Iron Ring (one of those kids books that's still good for adults), lots of Ursula LeGuin (particularly her early stuff). Terry Pratchett (probably best to get people started with a later novel like Thief of Time, Small Gods, Wee Free Men, or Making Money).
Fanfiction: Luminosity, HPMOR, most things alexanderwales has written (all good, maybe not all what you want), and the superman fanfic Veritas are all pretty recommendable to general audiences. Another contender would be author Forthwith's My Little Pony short stories (not that their longer stuff isn't also good, it's just not finished).
Thanks for your recommendations! I've edited my post to make it more specific, I am looking for content that promotes altruistic behavior, either fiction or nonfiction.
Hi Less Wrong,
I've got the opportunity to promote books and other forms of content to a largely teenage audience. I'm looking for some good book recommendations and recommendations for a limited amount of other media (websites, movies, etc) that will spread awareness of positive ideas and issues in the world, while still being entertaining to the target audience. HPMOR comes to mind, and although I don't think its main focus is to promote an issue to readers, it is an excellent choice because it is well ranked, promotes rationality which can directly help readers, and is completely free to read online. Recommendations don't have to be free, but that is a very important factor.
Thanks everyone!
Edit: Thanks everyone for your contributions. I realize I was being a little too vague, I think content that promotes ethical/altruistic behavior is mainly what I am looking for, either inspiring it with fiction or causing motivation for it with nonfiction. I am looking for free web content, although paid content recommendations are also appreciated. This is for a general youth audience, not a club or any sort of interest group.