JKR said that careful readers of HBP would find another Horcrux, which leaves me wondering what that certain Horcrux could be? As we know, the Horcruxes discovered most certain are the diary of Tom Riddle (destroyed), Marvolo's black ring (destroyed), the silver locket (possibly destroyed), Hufflepuff's cup (not destroyed), and three other ones that we don't know (since Voldemort himself is the seventh).
Harry Potter
Harry would be an interesting idea for a Horcrux, but ultimately, would not be feasible because Harry would have to die in order to destroy the Horcrux, and as Dumbledore said, Voldemort, even as a mortal, contains the same amount of intellectuality and power that he had before. If Harry died to destroy the Horcrux, no competant wizard would be able to destroy Voldemort, seeing as how the prophecy remains "neither can live while the other survives." Still, however, it would be a most interesting twist in the plotline- maybe neither would survive.
Nagini
I find this an interesting one as well, except for the fact that, as Dumbledore said, it would be unwise to use an animal as a Horcrux. Nagini has a mind of her own and will eventually die, so right there is a reason that making her a Horcrux is unlikely. I'm sure that Voldemort would be that remiss in putting the Horcrux in something that isn't immortal or inanimate, but we can't rule it out.
Black Goblet
This is the silver goblet with the Black family crest on it that Mundungus Fletcher had stole from Grimmauld Place. I can't be too sure whether or not this is a Horcrux but if, and only if, Mundungus is not who he appears to be (i.e. loyal member to the Order), then the possibility lies that he could have been stealing it back from Grimmauld Place to give to Voldemort before Harry goes back there. It might be farfetched and the evidence isn't so solid that you would bet your life the goblet is a Horcrux, but it kind of does make sense, if only a little. Plus, it was the only historical artifact really mentioned besides those of Voldemort's interest.
What do you think?