If you've read the Harry Potter books, you know that they read like mysteries, and that they tend to have surprising twists at the end. These twists, of course, are supported by subtle clues throughout the book, but they're hard to see coming.
In the latest book, Harry Potter and the Half-Blood Prince, everyone is talking about Dumbledore, Snape, and the other principals with regard to what's going to happen, but I want to put forth an alternate theory about what is going on in the J.K.Rowling series. It concerns one of the overlooked minor characters: Ginny Weasley.
Warning: there be spoilers ahead!
We know, from "Chamber of Secrets," that Ginny had been solidly under Voldemort's control at one point. Ostensibly, the destruction of the Diary freed Ginny of that control. But did it? There is ample evidence that Ginny remains a thrall of Voldemort. Consider:
* Ginny's personality has changed radically. Where before there was a shy, uncertain girl, now, suddenly, there is a headstrong, confident girl. This girl freely admitted to engaging in a long campaign of deception to catch Harry's eye - at the end of book six, she even says that Hermione advised her that Harry would "take more notice if [she] was a bit more - [her]self." That pause may not be a simple hiccup of emotion, but Ginny stopping herself from saying something that might betray her nature as that of Voldemort. And indeed, Harry DID take more notice when she was acting more like Voldemort (headstrong, confident, impetuous) than Ginny (quiet, shy, skittish).
* In the sixth book, the Weasley brothers opened their joke shop, which included a wide variety of love potions and other enchantment charms which sold briskly to the female students of Hogwarts. It was also revealed that the Weasley Brothers were also creating higher-power, "weaponized" charms and trinkets for use by the Ministry and other high-end clients. As a blood relative to the Weasley brothers, Ginny could have had easy access to much more powerful and subtle enchantment magic than the likes of Romilda Vane. Furthermore, it was clearly established that love potions were easy to sneak through security, so she could have easily taken them into Hogwarts to use against Harry.
* Ginny was in the D.A. with Harry, and so had intimate knowledge about him AND exactly what he knew about the Defense against the Dark Arts. She would have known what spells and potions Harry would have been able to discern or counter, and which ones he would not.
* As we saw with Ron falling victim to a love potion, the effect comes on suddenly, and is very obvious to friends what has happened. To create a more gradual effect, Ginny would have had to slip Harry smaller doses regularly over time. But Ginny had regular access to Harry and his personal possessions. Between being a Gryffindor (thus having access to the Gryffindor tower), being a Weasley (thus being "in" on the goings-on of the group), and being on the Quidditch team, it would have been a simple matter for her to slip him a dose of love potion whenever it was required, either a large emergency dose, or small doses over time so that he or his friends do not realize he is being slowly charmed.
* Harry found strange thoughts jumping unbidden into his head, as if from some other source. Harry's infatuation for Ginny manifested rather suddenly, and it surprised him. This came at a time when many other girls happened to be vying for his attentions, and it came just in time to stave off his direct interest in any of them.
* Harry's sudden infatuation with Ginny doesn't make sense. First, she's pretty young for Harry, especially when the people around him are hooking up with their own graduating class (most notably, Ron and Hermione). Harry never gave her a second thought before - indeed, she was a bit annoying. And he had ample time to get infatuated with her during their frequent Dumbledore's Army (DA) meetings, but he did not, opting instead for Cho Chang. And then, Harry started staying with the Weasleys, and came to think of Ginny as more of a sister, making it VERY out of character for Harry to suddenly have little unbidden thoughts come squirting into his head. Clearly, he was being worked over by subtle magic.
* Dating Ginny posed inherent dangers which Harry strangely seemed ignorant or dismissive of. Surely it would be inappropriate for a Quidditch team captain to date a member of the team, since concerns could be raised about both preferential treatment and coersion. Second, Ginny had become strangely popular with the boys practically overnight, and Harry's already enviable position (being the Quidditch team captain, the "Chosen One," etc.) could have sparked an attack from a disgruntled classmate. Finally, though, although he worried about Ron's reaction should he start dating Ginny, Harry did not even consider that the friendship that he, Ron, and Hermione shared was key to his very survival according to Dumbledore. Had he shattered Ron's friendship, he'd be imperiling himself, but he seemed somehow not to think of this sort of consequence. Perhaps this was Ginny's ultimate goal - to drive a wedge between Harry and Ron, to peel Harry's friends away from him.
* When Harry asked Luna to Slughorn's party, he did it suddenly, and was surprised that he had done it, almost as if he had temporarily lost his own will. Furthermore, Luna was a "safe" choice for Harry - her quirks made her an unlikely long-term match for him. Ginny, close as she was to Harry, and having had ample time to watch his reactions to Luna during their D.A. meetings, would have known that Luna offered no romantic potential for Harry. This effectively blocked Harry from asking out a girl he might come to really like. And somehow, it never even occurred to him to go with Hermione, despite the fact they were discussing not having anyone to go with. If he was simply looking for someone to quell the love potion attempts on him with, why not Hermione? Ginny couldn't risk that the romantic overtones of the party would push them together, and she wanted Harry separated from his friends, not closer to them.
* Ginny was privy to the fact that Dumbledore was giving special lessons to Harry, but not about what. Occupying Harry's mind with thoughts of Ginny would interfere with whatever it was that Dumbledore was teaching him or instructing him to do (and it worked). Also, there was a good chance that Harry would tell a girlfriend about what Dumbledore was teaching him, something that Voldemort would really want to know.
* On the day Harry took the luck potion, Ginny could reasonably assume that the Scoobies would be in the Gryffindor common room, since everyone would be discussing the results of the Apparition testing. Reasoning that Harry might be in low spirits because his friends could take the Apparition test while he could not (and correctly guessing that at least Hermione would pass), Ginny had a good opportunity to seal the deal with Harry. All she would need to do was to fabricate an argument with Dean and turn to Harry for solace. But when Harry took the luck potion, he narrowly missed that scene, passing by them just as Ginny started up a fight with her boyfriend. The reason for the fight was particularly lame ("he tries to help me through the door!"), so it was clearly meant to be a public break-up in front of Harry.
* Harry's brief contact with Voldemort left him able to tap into Voldemort's thoughts and feelings. Ginny was mentally controlled by Voldemort over the course of nearly a year. There's no way that she could have come out of that experience without an even deeper, permanent connection to Voldemort.
* When the Diary was destroyed, one of Voldemort's Horcruxes was destroyed. Or was it? When the Diary took over Ginny so completely, it's possible that the Horcrux passed over to her - after all, Dumbledore did say that living creatures can serve as containers for a Horcrux, and we have seen Voldemort cohabitating a body with Professor Quirell. Apparently, the Diary was smart enough to not only communicate with Potter and Ginny, but also to plot ahead, deceive, and perform magic. Moreover, this may have been its intended goal. Dumbledore said that Riddle desired an artifact of Gryffindor for a Horcrux, but that he had been denied access to any. Perhaps he then built the Diary to lie in wait for a Gryffindor to take possession of it, and then transfer the soul fragment into that poor child. Then, Voldemort could act through that student, and could always claim to have been struck by an Imperius curse if he got caught.
* When Harry is breaking up with her at the end of the book, he tells her that the reason is because he doesn't want Voldemort coming after her. Her response is that she doesn't care. This is unlikely if Ginny is innocent, because she's already tasted what a fragment of Voldemort's soul could do to her, and has seen many of her brothers nearly killed by Voldemort. Having only dated Harry for a month or so, it's unlikely she'd be so flippant about giving her life to be with him. She'd at least hesitate. The only explanation is that she knows that he has nothing to fear from Voldemort.
* Whenever we see Ginny at home, we also see that the Weasley clock points everyone to "Mortal Danger." While we are led to believe this is the general state of affairs for everyone, it's unlikely they are all in "mortal danger" all the time. That's like having a terror alert system with only one level. Instead, perhaps it is a specific reference to the fact that Ginny has turned and is ready to kill with the Avra Cadavara anyone who figures out that she's a thrall of Voldemort. Because her family knows Ginny best, they are in the most danger of being cursed.
* On the day of the Death Eaters' attack on Hogwarts, as the theory goes, the pub owner in Hogsmeade, Rosmerta, was under an Imperius curse, and that she was the one that noticed Dumbledore leaving and alerted the Death Eaters to come to Hogwarts. But why would she be under the Imperius curse? Voldemort would have no reasonable expectation to think that she would witness when and where Dumbledore apparates away, and Malfoy would have no use for her after a failed murder attempt with the necklace. More likely, Ginny was still a thrall of Voldemort, and when Harry re-activated the DA and told them that he was going off with Dumbledore, Ginny saw her chance and alerted the Death Eaters herself. To throw suspicion off of herself, she implanted in Harry's mind (using whatever tactic she used during her seduction of Harry) the idea that the poor lady was under the Imperius curse. (Also, it was probably Ginny, not Rosmerta, that planted the deadly necklace from the ladies' room.)
* During the fight with the Death Eaters, Ginny was locked in battle with a Death Eater who would throw spells around wildly instead of aiming them directly at her. Was this the effect of the luck potion? Or complicity? The luck potion would have made them near misses, not hugely wide shots as the text implies. Furthermore, Ginny, despite her assailant's wide-open flailings, apparently refrained from cursing him, probably because Ginny did not actually want to hurt the Death Eaters.
* How did Malfoy know about the Room of Requirement? None of the students were supposed to know about it. Ginny was one of the few who did, being in the DA. She could have easily informed Malfoy of the room. And how did Voldemort let Malfoy know that he had been chosen for a task, and how was Voldemort putting pressure on him to complete his task? If security was tight enough to keep Voldemort out, he must have had another insider (besides Snape, who apparently could not communicate with Malfoy) with which to put the screws to him: Ginny.
So the evidence is clear: Ginny is Voldemort's double-agent inside the Scooby gang at Hogwart's, and they have no idea. Sparks will fly in book seven!