Defect by Kikkimax. It's a Criminal Minds crossover - PG, Gen (just the way I like 'em).
I don't even watch Criminal Minds. I don't know the characters. I don't know what they look like. I don't know the dynamic between those characters....but it actually didn't matter. The fic works just great as an Outsider POV on the Winchesters (with a few Insider POVs interspersed.)
It serves as a slight AU to the end of S3, picking up sometime before Dream a Little Dream Of Me, and coming up with an alternate ending to the Dean's Deal storyline.
It's got a sequel, but the sequel isn't as compelling as the original fic, in my opinion.