My all-time favorite J2 AU fics (in no particular order):

But Then My Homework Was Never Quite Like This (http://thenyxie.livejournal.com/481145.html) by thenyxie
(be sure to get all the sequels/codas/timestamps)
Summary: Jared is eighteen and in his senior year at a private school in San Francisco. He is coasting pretty on his parents generous school donations -- until his seventh period Physics teacher gets replaced with the maddening, infuriating, gorgeous Mr. Ackles. Jared has gotten by all his life on his money and his charm, but Mr. Ackles seems to think he needs some... lessons in self-control. The journey their relationship eventually takes them on turns out to be far more than either of them ever bargained for.
Reccer’s Note: Truly epic! And honestly the best erotica I have ever read, including professionally published works. Truly, this story is the very definition of ‘erotic’.

My Heart Don’t Beat The Way It Used To (http://raina-at.livejournal.com/315214.html) by raina_at
Summary: Jared and Jensen have been best friends forever, and nothing’s going to ever change that, not even senior slump, or college anxieties, or even Jared getting an annoying cheerleader girlfriend. Or so Jensen thought.
Reccer’s Note: This fic has kinda haunted me since I first read it; I find myself drawn back to it over and over again. Jensen’s longing, his self-realizations, his struggles to regain balance, really stick with me. And the happy ending, despite feeling like it is in jeopardy the whole time, also feels inevitable once it arrives.

Can’t Point The Way To Your Heart (http://belyste.livejournal.com/15726.html) by belyste
Summary: In the vein of movies like Hitch -- Jensen's an unofficial advice guru, and Jared's his latest project. Except while Jensen's supposed to be helping Jared end up with the woman of his dreams, he falls for him instead.
Reccer’s Note: Jared is sooooooooooo funny in this! Clueless and adorable and romantic. Jensen is a fantastic foil for him; not stuffy, really has himself together. Most times, authors tend to write the ‘serious’ character as a total boring stick in the mud, but not here; Jensen is sharp-witted and cleverly humorous, not some big ball of angst. Without giving anything away, there is a part that made me want to cry; I honestly got a little choked up. But nobody dies tragically, and it’s a total happy ending, so it’s completely worth it! This is Romance at its best. I found myself falling in love with these endearing characters, and really rooting for them.

The Ones That Can Swallow You Whole (http://embroiderama.livejournal.com/298657.html) by embroiderama
Summary: A near-drowning has left Jared deeply afraid of water -- a big problem considering he lives in a beach town. Jensen is a sailboat-builder who has survived a few things himself.
Reccer’s Note: This one is about helping each other heal. It focuses on the recovery, rather than the trauma. It is very well written, struck just the right balance of emotions, didn’t go overboard on the angst, while still being satisfyingly dramatic. It just totally gave me warm fuzzies, the whole time! My cheeks kinda ached a little from grinning so much. I am such a sucker for a good J2 AU romance! *happy sigh*

Then Again I Just Might (http://community.livejournal.com/saucyminxfix/16111.html) by cha and anyothergirl415 (aka Charlotte and Miss. Brie)
(be sure to get the sequel too)
Summary: It's not totally weird to be in love with a person's voice, is it? If it is, Jared's pretty much okay with being weird.
Reccer’s Note: Stuttery, painfully shy Jared is the most adorable thing ever! And Jensen is the awesomest, coolest, sexiest boyfriend. This fic is sooooo romantic -- and insanely hot! The two of them are just the adorablest adorables who ever adorabled. Plus, the soundtrack is fantastic.