I posted this theory on another forum, but wanted to see what you guys had to say about it (see how I value your opinion? Cool, huh?):
RE: The Seven Horcruxes
Okay, so five we know about, and at least two have been dealt with. The diary in CoS has a fang through the front cover, the ring that screwed up Dumbledore's manicure has been destroyed (in a way we don't get to find out about), the locket, which has gone AWOL and may or may not already be destroyed, the goblet of Hufflepuff, which is currently stored in an undisclosed location, and Voldemort himself. So we're left with two unknowns. Given what we know about Voldemort, and about how his brain works (collecting trophies, sticking them in places that mean a lot to him), it can be surmised that the other two horcruxes are most likely things left behind by Ravenclaw and Gryffindor. Now, as far as the Ravenclaw object goes, I presently have no theories (though I may move on to that after this one), but I do have an interesting idea on something Gryffindor left behind:
Below is (most of) my original post, reposted for your perusal and fine-tooth combery.
"A theory I quite liked (which I have just found a little evidence for) is what if one of the horcruxes is the Sorting Hat? I figure the location of one of the horcruxes has to be in Hogwarts, because Riddle was a great wizard even then and no doubt achieved a great deal in those hallowed halls, which supports it as a place 'of great importance' for him. Plus, we know he loved the place. So it's not that much of a stretch to think there's something in there waiting to be found. (Note: Although the diary was used inside Hogwarts, it was brought in from outside, so I don't count that as a Hogwarts related Horcrux).
So why the Sorting Hat? Well, it's book seven. No more need for it afterwards, because there'll be no need for Rowling to announce new pupils (aside from the new first years, but I figure they'll be settled in before Harry figures all this out), even if the Potter universe carries on after the end of the 'Harry Potter Saga'. And, from an 'inside the universe' perspective, it could well be that the hat's constant talk of "all the houses banding together" is finally realised when the houses are removed, making Hogwarts one big, happy family. Further support for this theory could be that Hogwarts is re-opened, but few people attend (probably under direction from worried parents), making catering for four houses an impractical impossibility. The added friction created by having four previously competing houses (and with no small amount of animosity between some of them) would make for some dramatic encounters, I'm sure.
And the most compelling bit of evidence? Why, the ownership of the hat: one Godric Gryffindor. Something Dumbledore seems to have overlooked, as he states "the only known relic of Gryffindor [the sword] remains safe". Now, it may be that after the four houses bewitched and put brains in it, it is no longer considered a 'relic of Gryffindor', but somehow I don't think the founders messing around with it diminishes its overall Gryffindor-ness.
Finally, I haven't found the passage yet, but I'm pretty sure somewhere we're told about Riddle's sorting, and how the hat didn't even need to be placed onto his head before he was chosen to be in Slytherin. Whether this was one of those times that JKR threw in a bit of seemingly innocuous bit of information which has a lot of bearing, or whether it is something Riddle would deem significant enough to place a seventh of his soul into, I don't know. But if he was desperate to get a relic from each Hogwarts founder as I think he is, and given that the sword (arguably the ideal Gryffindor relic) is pretty well protected, the Sorting Hat would be a pretty good substitute."