I was trawling the depths of the HP section of ff.net when I cam across thia fic. It takes the old and tired trope of "Harry Potter has a twin", and actually manages to do something with it that doesn't result in a Mary Sue. The twin- Katharine Potter -doesn't muck with the story (it starts out in first year), isn't all-powerful, and actually has (to me, anyways) a well-developed character that doesn't leech off Harry. Everyone's in character, there're a few ocs that are surprisingly well-written, and I found it a refreshing change from the normal drivel that has this tired trope in it.

Unfortunately, the fic's been on hiatus for a while, but at least someone made a rather good attempt at making a story like this believable.