Seconding Suiteverse, though by now everybody and their (gay incestuous) brother has probably read it....but if you haven't, it's a must if you like dark Sam goes evil for Dean stories.

You should also check out sammessiah if you haven't yet. It's a comm devoted to evil Sam, and many of the fics collected there have sweet little Sammy only turning evil because he's misguidedly trying to save Sam. (Though sometimes not...)