Mockingbird (http://hiyacynth.livejournal.com/218645.html) by hiyacynth is a 10 chapter Sam/OFC fic, with this summary: Dean frowned and shook his head. Even if he wanted to, how could he start to explain what Sam was these days? He wasn't sure himself. He knew what Sam had been before the war. He had a pretty good idea of what Sam'd been during. And he knew Sam wasn't either of those things anymore.

Probably my favorite long het fic in SPN, if you don't mind that it goes AU near the end of season 1. And Dean is in it plenty; I wouldn't enjoy a fic that is Sam or Dean; I want them both!