With everyone going Pottermad, I figured I'd rec a fic that's probably already been recced and read a thousand times:
The Wendell That Wasn'tHarry is haunted by Snape's ghost. And Ginny is a fiend. (Not literally.) It's humourous, funny, delightful, and LOLworthy all over, and if you know anything about the HP books, you really must read it. :)