The following things will happen, according to the timeline and practices set by other books or things Jo Rowling has said:

Harry will turn 17. He will be allowed to perform magic outside school, and obtain an Apparition license.

Harry and his friends would be in his seventh year at Hogwarts School of Witchcraft and Wizardry - however, we don't know if he will attend class at all, or if Hogwarts will be open.

Harry will return to the Dursleys one last time.

He plans on starting his Horcrux hunt in Godric's Hollow. Ron and Hermione will accompany him, but the trio will first attend Bill and Fleur's wedding.

Seventh-year students will take NEWTs, the most difficult of wizarding tests.

The last chapter will contain details of what will happen to survivors. The last word is still "scar" (but may easily change).

There will be no Quidditch.

Harry and Ron will never read Hogwarts, A History. Never, Hermione. Ever.

There will be the final, climactic confrontation between Harry and Voldemort. (Harry's going to win, Harry's going to win, Harry's going to win...oh, sorry. That's just our opinion. Sort of. In a not-very-unlikely-opinion sort of way.)

Interview notes: We have all been obsessing about J.K. Rowling quotes for years; many of them haven't been clear on which book they reference. As there's only one book left, there are certain facts we can pull out of her interview quotes (using her Web site and Quick Quotes), which give us yet more clues as to what will happen in book seven.

The locked Ministry room may resurface. "No comment," said Jo, when TLC/MN asked her if we'd see it again.

We'll hear about Dumbledore's other letters to Petunia. Jo said, on her FAQ section:

What did Dumbledore's Howler to Aunt Petunia mean? ('Remember my last'?) ...Dumbledore is referring to his last letter, which means, of course, the letter he left upon the Dursleys' doorstep when Harry was one year old. But why then (you may well ask) did he not just say 'remember my letter?' Why did he say my last letter? Why, obviously because there were letters before that...
Now let the speculation begin, and mind you type clearly, I'll be watching...

P.S. It has been suggested that I am wrong in saying that Dumbledore's last letter was the one he left on the doorstep with baby Harry, and that he has sent a letter since then concerning Harry's illegal flight to school. However, both Dumbledore and I differentiate between letters sent to the Dursleys as a couple, and messages directed to Petunia ALONE. And that's my final word on the subject - though I doubt it will be yours :)

There is yet more to Petunia than we know. This may relate to the above, but J.K. Rowling said the following at the Edinburgh Book Festival in 2004; since it wasn't delivered upon in book six, it remains for seven: "You might have got the impression that there is a little bit more to Aunt Petunia than meets the eye, and you will find out what it is. She is not a squib, although that is a very good guess. Oh, I am giving a lot away here. I am being shockingly indiscreet. "

Umbridge will return. "It's too much fun to torture her and not have another little bit before I finish." (TLC/MN interview)

There is yet more to come about Rita Skeeter: Also at the Edinburgh Book Festival: "I actually quite like Rita. She is loathsome - morally, she's horrible - but I can't help admiring her toughness. She is very determined to do the job and there is something quite engaging about that. There is more to come on Rita."

We will discover what Dudley saw when the Dementor approached in book five. Someone asked this in the World Book Day chat; Jo said, " Ah, good question. You'll find out! "

The two-way mirror from book five will likely resurface. (World Book Day Chat)

We will find out what happened to Sirius's flying motorbike, "but the real sleuths among you might be able to guess." (World Book Day)

Krum will appear. (World Book Day)

Grindelwald as something to do with the remaining plot. (TLC/MN interview)

Sirius may appear again, though in which form is uncertain. The infamous question from the World Book Day chat read, "If we ever see Sirius again, what form will he be in?" Jo said, "I couldn't possibly answer that for fear of incriminating myself."

We will find out a little bit more about Harry's grandparents. (World Book Day)

Harry's eye color (or something about his eyes resembling his mother's) is very important to this book. (Many chats; World Book Day, most recently, but there's also this nugget from the Boston Globe from 1999: "But he has his mother's eyes and that's very important in a future book."

Wormtail's life-debt to Harry will come into play. (World Book Day.)

James and Lily Potter's professions/more about their lives will be revealed. (AOL Chat, 2000) This is "important to a future plot," as Jo revealed to Scholastic in Feb. 2000.

The gleam of triumph seen in Dumbledore's eyes at the end of book four is still enormously important. (TLC/MuggleNet Interview, July 2005, and others.)

Harry may time-travel again. Jo has said "Not telling!" to this question, usually the kind of dodge that means there's something related to it upcoming in the books. (AOL Chat, 2000)

A non-magical character will perform magic in "desperate circumstances." Jo said this would happen back in 1999, in a Barnes and Noble chat, but it has yet to occur. She said, "There is a character who does manage in desperate circumstances to do magic quite late in life, but that is very rare in the world I am writing about."

Mr. Weasley's car will appear. Jo said in a 1999 Barnes and Noble Chat, chat that we "will hear from Mr. Weasley's car again, but yet again, I'm not telling you how." Only one book left!

Snape Snape Snape, itsa Snape, alwaysa Snape... from "The Connection, 1999":

There's an important kind of redemptive pattern to Snape. He, um, there's so much I wish I could say to you, and I can't because it would ruin. I promise you, whoever asked that question, can I just say to you that I'm slightly stunned that you've said that and you'll find out why I'm so stunned if you read Book 7. That's all I'm going to say.

One of Harry's classmates will become a Hogwarts teacher. Also revealed in The Connection. C'mon, Jo, we know Neville takes over for Sprout - which means he won't die, right? Please? Please? ... Fine, be that way.

A huge piece of information is coming about Lily Potter. Also revealed in The Connection. "...you'll find out something incredibly important about her in Book 7. But I can't tell you what those things are so I'm sorry, but yes, you will find out more about her because both of them are very important in what Harry ends up having to do. "

We will find out why some witches and wizards become ghosts and some do not. (Scholastic, Feb. 2000)

The "Godric" in "Godric's Hollow" is not insignificant. Lizo conducted a four-part interview in 2000, in which the following exchange took place:

The significance of the place where Harry and his parents lived - the first name... Godric Gryffindor. Very good, you're a bit good you are aren't you. I'm impressed. You're not going to tell me but... My editor didn't, I said to her - Haven't you noticed the connection between where Harry's parents lived and one of the Hogwarts houses? And she said no, no - I'm not being rude about Emma, she's a brilliant editor, the best ever. But no she didn't pick that up either, you're a bit good you are.

And to assist you further in your clue-finding quest, here's a list of rumors J.K. Rowling has debunked recently:

Aunt Petunia is not magical, nor will she ever perform magic.

Lupin will not be back as a Defense Against the Dark Arts teacherin book seven.

Peter Pettigrew's silver hand will not be used to kill Remus Lupin.

J.K. Rowling will not write a book about Lily and James when she's done writing Harry Potter.

The Sorting Hat is not a Horcrux.

Mrs. Norris is not an unregistered Animagus.

Harry is not a Metamorphmagus.

Dumbledore is not Ron or Harry from the future.

Nicolas Flamel will not teach potions at Hogwarts.

Luna and Neville will not "hook up."

The Lestranges were not sent after Neville to kill him.

Neither Voldemort nor Dumbledore are Harry's close relatives.

Lily was never a Death Eater.

Lily Potter is not alive.

Crookshanks is not an Animagus.

Neville is not Pettigrew's son.

Lupin does not have a twin.