http://www.harrypotterfanfiction.com/ -> "we currently hold over 60,000 stories"
The search page must truly be seen to be believed: http://www.harrypotterfanfiction.com/storytype.php Metadata search as you've never seen it before!
Sturgeon's law is in full effect, of course, but don't forget its fuzzy-logic contrapositive: 10% of everything isn't crap. I can't say I've drunk deeply from this well, but I've read a couple of things that makes you wonder why the author bothered to "steal" the setting at all, when they could quite clearly make their own. (Though it also shows the value of editors and their mighty Red Pens of Cutting Out About A Third Of The Text.)