Potter, Harry.

See, when you are writing a series of books, and you're approaching the final one, and the penultimate book has been released, you're going to produce a LOT of speculation on how it's all gonna end.

Even if it is a "children's" book series.

Some of the following came from my brain on it's own. Some I discovered on the net from other people's brains. (Frankly, other people's thoughts amaze and/or disturb me.)

Spoiler time!
[begin speculation on the Harry Potter series, includes spoilers of all the books including Harry Potter and the Half Blood Prince]
Where to begin?

A lot of this speculation is going to be wild, way out there stuff that I have no hopes of being true. (I'd be disappointed if some of the things did come to pass. You'll see.)

At the end of Harry Potter and the Half Blood Prince, Dumbledore has been murdered before Harry's eyes, by Professor Snape. There has been much speculation as to whether Snape (who has throughout the series been distrusted by Harry, and  completely trusted by Dumbledore (who never gave a reason for trusting him)) is on the side of good or of evil.
When I first read the book, my initial thought was that it was surprising that JK Rowling had opted to finally choose a side for Snape, after doing the whole "is he good, is he bad" thing through the series.
Since then, I've thought about it, and seen some pretty darn convincing evidence that the side Snape is on is still that of the good guys.
And while I'm 98% sure that that will end up being the case, there's still that 2% of doubt that keeps reminding my brain that it's there to be dealt with.
So, here are the cases for why Snape might actually be evil.
1) Harry has distrusted Snape from the get-go. Harry is the protagonist, and to have him wind up being wrong seems ...wrong.
[Snape is Good counter point - Dumbledore has trusted Snape from the get-go. To have Dumbledore be wrong also seems wrong. Although Dumbledore has been wrong before. (see: Not telling Harry all the information, believing that Crouch Jr was Mad Eye Moody) Also...Dumbledore might not be all that he was made out to be. But more on that later.
I guess it really comes down to who was right - Dumbledore or Harry?
Harry has always been a good judge of character - he disliked Draco right away, knew Ron and Hermione were good - so why be off about Snape?
Dumbledore, on the other hand, has always been illustrated as highly intelligent. But, especially in book 6, as perhaps too trusting.

A large component in the "Snape is Good" argument is that Dumbledore knew about the Unbreakable Vow that Snape made. I think he probably did. But that doesn't mean that Snape is a good guy. It just means that Dumbledore knew that either Snape or Draco was going to attempt to kill him.

Perhaps, Dumbledore, in his wisdom, knows that Snape is the bad guy, but is okay with that, because he knows that it will come into play somehow. And his "I trust Snape" statement is really meaning, "I trust Snape to play out his role in the bigger scheme of things".

I'm not explaining myself very well.

Another doubt as to why Snape is good.
It seemed that Draco was wavering when Snape and the Death Eaters came into Dumbledore's office.He had also been under huge amounts of stress to commit the act of murder. It seems that Draco is realizing that being a Death Eater is as glamorous as he had believed. This might mean that Draco is going to deflect to the good side.
Now, if Draco is redeemed, and RAB ends up being Regulus Black, AND Snape ends up being a good guy...it's like, dude. Is ANYONE truly evil in this book? Maybe Voldemort will wind up being a good guy too.

Point #3) Making Snape one of the white hats moves too much of the focus of the series onto him. The books are supposed to be about Harry Potter, not Severus Snape.

#4) I want Snape to be evil.

Of course, I also want him to be good.
I want to eat my cake and have it too.

Before I leave from the Snape-talk, a theory that I told Amy I'd mention in this - Snape and Harry's mother, Lily. It's been postulated that Snape liked Lily. I can see this being brought to light in book 7. The wild theorist in me thinks that, hey, maybe Snape is even Harry's true father.
Except that it's been stated over and over again that Harry looks so much like James. (Perhaps Lily used magic to make Harry have James' appearance? Hmmm. Okay, definitely moving into fanfic territory here.)
But, yeah, if Snape had a crush on Lily, it would explain (in part) why he hates Harry so much. Harry is the result of his unrequited love. Plus, Snape is responsible for Lily's death.
It'll be interesting to see what direction JK Rowling takes with Snape. And how much redemption - if any - she gives his character.

Harry the horcrux.

I hate this theory.

The idea that Harry will have the last piece of Voldemort's soul in him just sucks. While there is some compelling evidence to support the theory - the Sorting Hat wanting to put Harry in Slytherin, Harry being a parseltounge, the connection between Voldemort and Harry in the earlier books - it's just too...wrong, somehow.
First, Dumbledore would've known. And he would've found a way to either remove the horcruxness from Harry, or perhaps have Harry destroyed. (Yikes. The killing of an innocent baby? I think Dumbledore would've done it, though, if it meant the complete death of Voldemort.)
Second - if Harry is Voldemort's last horcrux, it means that if Voldemort is to be completely beaten by the end of the series, that either
a) Harry has to be killed to
b) some sort of way to remove the horcrux from Harry's body has to be done [which could be done, it's a fictional story and we don't know much about horcruxes, but I'd take issue with this being the solution since, like I said, Dumbledore would've found this out long ago]
c) Harry won't defeat Voldemort

There is a semi-related theory that only part of Harry is the horcrux - say his eyes, or perhaps the scar - and I can buy that a little easier, since that would mean that Voldemort's death (or total death, rather) would leave Harry alive - but blind or critically injured.  If she goes the Harry is a horcrux route, I think that something like this might be the best way to go. But I'd prefer that it not be touched upon at all.

Speaking of horcruxes...the locket at the end and RAB...Dumbledore was convinced that the horcrux was there in the cave. Which leads me to believe that either - the note is false, and the locket IS the horcrux, or perhaps Dumbledore DRANK the horcrux. Meaning that Voldemort put the horcrux in the potion that Dumbledore drank, rather than in the locket in the potion. Because, really, how did R.A.B. put the potion back after destroying the original?

Blah. I'm done. I've had a hell of a day, and I don't really want to talk Harry Potter anymore. I've lost my train of thought, and I'm not sure what my point(s) is/are/were. But, briefly, crazy theories:

1) Dumbledore is evil. From Book 4 when Harry mentions that Voldemort can touch him, he sees a look of triumph on Dumbledore's face. I've got other evidence to back this up, but if it turns out that Dumbledore IS evil, it would be more of a disappointment than a shock.

2) Hagrid is evil. Hee! Never see that one coming. Similarly, that Hagrid is the last horcrux. There's no way it could happen, but it would be a shock.

3) That Book 7 won't be the end. Or, rather, that the confrontation between Harry and Voldemort won't take place. The books are supposed to chronicle Harry's years at Hogwarts. Nothing says that he'll actually destroy all the horcruxes and face Lord Voldemort before year 7 ends.

4) Harry will be the new Defense Against the Dark Arts teacher. I suspect that this one might happen. This will be how they get Harry back to Hogwarts for the 7th book. It's been hinted at - he was teaching in book 5 - and it makes a lot of sense. Plus, it would just be really cool.

So, yeah. Not the great Harry Potter post I had intended, but rarely do things turn out as planned, eh?
[end speculation discussion]

As always, if you're feeling spoilerphobic, don't read any of  the comments.