"The Misadventures of Draco Malfoy, Ferret" by Saber ShadowKitten

Just ... read it.

http://www.fanfiction.net/s/2690275/1/The_Misadventures_of_Draco_Malfoy_Ferret

"Harry Potter and the Slytherin Spy" by Christine Morgan

What if there was another Hogwarts student in Little Whinging?

This fic, written/set right after Order of the Phoenix, is about Harry meeting a Slytherin girl and forming a friendship with her in his sixth year. It's fairly-straightforward and not excessively (wonderfully) violent at all. (Sarcasm.)

http://www.eskimo.com/~vecna/hp_spy_01.html

Morgan can imitate Rowling's writing style well, and there is something so satisfying about reading a fic you can confuse for the genuine article; especially when it gets to where Morgan is describing gruesome deaths in Rowling's simple, yet descriptive prose.

I'm not pretending this fic is without problems; it's focused on a female OC, which is a no-no to most in the fandom, though trust me when I say the writing is good enough you won't care. And there are a few canon things that would raise eyebrows - Kreacher's characterisation is all wrong in this fic, in my opinion, while I think Morgan's explanation of where Sirius was hiding during Harry's thirteenth summer is a brilliant twist. Morgan also abuses ellipses, and the ending seems a bit rushed and simplistic, but trust me when I say:
YOU SHOULD READ THIS.