Books. Lobsang beat me to what would have been my first suggestion, but two classics that come to mind are L. M. Montgomery’s Emily of New Moon (a bit darker and more mature than Anne of Green Gables, which is, of course, also very good) and Dodie Smith’s I Capture the Castle.
A pony. No, not really. But riding lessons are great for that horse-crazy age, and there are worse places for a kid to hang out than the barn.
StG
A gift certificate to a music/movie store. Let her indulger her favorite boy-pop fad du jour.
If you want to get ultra-lazy and do it without leaving your chair, Apple’s iTunes Music Store supports gift certificates electronically.
You obviously haven’t seen the spam I’ve gotten lately.
Does she like animated movies? If she does, The Iron Giant, Finding Nemo or Spirited Away DVDs are pretty good choices.
Not sure about the “wow” factor, though…what genres of fiction is she into? Has she read A Wrinkle in Time and its sequels? A Series of Unfortunate Events? Artemis Fowl?
Seventeen is a horrible magazine-- just training up girls to be readers of other ‘women’s magazines’ that don’t do anything but make you obsessive about your looks and get you buying makeup. 
More stress than fun. The later she gets into that scene, the better. Never is best.
I’d go with a lockable diary or a lockable box-- especially if she has siblings, a pretty box or book that nobody else can get into is always good.
Books are really cool, as someone suggested. I have a very unique t-shirt from a pet store, my 12YO cousin will likely wear it as a night shirt for the first year or so, and it has a horse going through a hoseshoe collectiont he same way a woman would go through shoes. I also will have a book on why dogs do the things they do.
Not quite the “wow” factor, but it goes with her interests and they’re both things she’d probably not find herself anywhere, so it would be “wow” in the sense of “where did you get this,” since I live about 30 miles away.
Don’t get her a book or book set unless you are very sure about her taste in reading. I always liked to read, so all through my teens my relatives would all get me the latest Stephen King or Danielle Steele or VC Andrews blockbusters each holiday. Those genres just didn’t appeal to me. Don’t get me wrong; I appreciated the thought, but I would have have really preferred a gift certificate to a bookstore.
What about a book on basic web design and an offer to help her get her own free webpage up and running? And I second just about every suggestion here. Really creative.
I laughed out loud at this!
Proof you are deserving of worship 
I think books are always a great idea! It’s hard to identify your tastes in books if you haven’t read a variety of them.
My advice: Look up a list of all the Newberry Award books and choose a few of those. It’s never too soon to begin building your library of classics!
Personal Favs:
A Wrinkle In Time - Madeleine L’Engle (and the companion books of this series)
Island of the Blue Dolphins - Scott O’Dell
The View From Saturday - EL Konigsburg
Holes- Louis Sachar
Walk Two Moons- Sharon Creech
Tuck Everlasting -Natalie Babbitt
Number the Stars- Lois Lowry
*The Newberry Medal Homepage http://www.ala.org/Content/NavigationMenu/ALSC/Awards_and_Scholarships1/Literary_and_Related_Awards/Newbery_Medal/Newbery_Medal.htm
FaerieBeth
The book The Egypt Game was an old favorite.
Or maybe The Mad Scientists Club, if she’s a Tomboy or a Nerd.
Thank you all for the great ideas! I’m warming up a bit to this shopping thing now.
And books are a given. The thing is, Santa gives toys, Mom gives books and clothes. (And Santa must be real, everyone knows Mom wouldn’t buy all that cool stuff. )
This made me laugh out loud - have a complementary 
smilie.
Well, I could never figure out what to get my sister at that age, either. And her Bday is on the 18th, so I had to buy two sets of present. Arg. So I always ended up getting her makeup and nailpolish in her favorite color.
Last year (14th Bday) I got her a purple moon mirror (Moon wrapped around the mirror with little flames), and a bag with worry dolls in them (what teenage girl doesn’t need some dolls to pour worry into?).
This year she’s gonna get her belly button pierced (same age as when I got mine, aww), so I’m getting her a bunch of navel gem barbells.
http://www.stupid.com/ has some . . . interesting gifts. Most of which I would love, lol. If she’s strange, it’ll be good times. Lots of strange toys.
Direct link to the toys: http://www.stupid.com/stat/TOY.html
I like the idea of getting her something a little grown-up. When I was 13, one of my aunts gave my a silky black bathrobe for Christmas. It made me feel so adult and sophisticated. (It wasn’t see-through or revealing, nothing like that.)
Jewelry and jewelry-making kits are also good. Though I don’t wear much jewelry now, I loved it as a young teenager.
One of the best Christmas presents I ever got was a copy of Our Bodies, Our Selves, also when I was 13, from my cool older cousins. It was incredibly informative and answered a lot of questions that I would have felt extremely awkward asking my mom. I still refer to it when needed. And my cousins wrote a sweet inscription in it, something like “For the awesome woman we know you’ll become.” Something to consider as a Mom gift.