A gift card is the best idea. To me a gift card says I care, but I realy don’t know what you want, and I want you to be happy with my gift.
Does he have a car? If so, things like Car Seat Covers, Steering Wheel Covers, Random Gadgety Things Which Plug Into The Cigarette Lighter, and so on are all affordable gift ideas for a 17 year old boy…
I would go with the Leatherman. I’ve got one and use it all the time. It’s a Man thing.
The Leatherman - or a nice knife - is a good idea.
I’d suggest you get him a book - something that you think he might enjoy reading, and appreciate that you gave him. You are a cool chick. Think back about what books you wish you had read when you were 17 but didn’t find until later. You know your family better than I, so you might be better able to identify what he might like or might have already read. Something edgy will have the cool factor.
A couple of suggestions: Basketball Diaries; anything by Hunter S. Thompson; most anything by Harry Crews especially Gypsy’s Curse, Feast of Snakes, Knockout Artist.
Or, given his interest in music, a nice “reference-type/coffee table” book along the lines of an Encyclopedia of Rock and Roll. Should be tons of neat options in the Music section of your local megabookstore.
Write something nice in it, and he’ll have cause to remember you fondly for years.
I strongly recommend against gift cards. Too impersonal. If you don’t want to shop, just give him cash.
You could always buy him a radar-detector, but that might be asking for trouble. Still, I have to go with the concensus here. Know any girls who look like Rebecca DeMornay? 
No.
A really, REALLY good Swiss Army Knife works/lasts better!
I’ve owned both, & I’ll never go back to a Leatherman.
When I recommended Leatherman, I was going off of what I had heard. Many people like them, but I have never owned one or had a need for one.
Similarly, I would recommend against huge multi-tool Swiss Army knives. Too bulky to comfortably carry, and he’ll never use the majority of blades.
I’d strongly recommend simply a decent quality folder. Better to have one quality tool that you will enjoy carrying and using, over a number of tools that will work less well and be inconvenient.
A Spyderco Delica is a fine knife just about anyone would be happy to carry.
http://spyderco.com/catalog/details.php?product=209
I think the blade under 3" is legal in IL, but you might want to check. You can get nice folders with blades under 2" that are just as useful for nearly every purpose.
Here’s a DJ store in New York city. The site doesn’t say whether or not they sell gift certificates/cards, but there’s an 800 number you can call to ask.
I would also recomend cash over gift certificates. It seems like whenever I get a Best Buy gift certificate, the thing I want most immediately goes on sale at Circuit City.
Another variation of the gift certificate theme - I used to get the “only-met-twice-in-my-life- hard-to-buy-for-step-nephew” on my list ticketmaster gift certificates. They can be used to go to concerts or sporting events - always a big hit.
Failing that, there’s always Chicken Soup of the Teenager’s Soul.
A nice, leatherbound King James Bible. Really. Church is full of girls.
He might prefer a nice leatherbound girl.
Tabby
And who wouldn’t!!
What about video games? Like… xbox stuff.
Does he have an iPod? Maybe iPod accessories… like a car hookup or a holder that straps on his arm
A cool pocketknife might be a hit. A lot of the boys at my school, after school, like to whip out their knives and compare them and sharpen them and stuff. As far as I know, none of them are into actually USING the knives for anything but opening their chips like barbarians. The girls like to ooh and aah over the boys’ knives as well. “ooh, that’s sharp! I should get a knife too. Or a boyfriend who has a knife. I’m just a little tiny girl and I need protection on the big bad streets of my suburb” eyelash bat
Magazine subscriptions are good becuase they keep going all year. I’m not sure what kinds of magazines boys read, though. I’m guessing not Seventeen or Pointe.
Also, subscriptions to websites are fun presents. I did a few paid livejournal and deviantART accounts this past year. Again, I don’t know what kind of websites boys like (other than video game sites and porn) but it’s a beginning of an idea, I guess
depending on how awesomely geeky he is, he might like something gross like chocolate covered bugs or weird japanese candy.
Almost anything from http://www.stupid.com would probably work. Stuff that will make him laugh and then he’ll take it to school and show all his friends and they’ll all talk about it. “Joe has a little Jesus doll who farts fire! it’s so cool!”
why are boys so weird?
And the porn in there is not too bad, either.
This is a touchy one. First of all, he’s turning 17 (not 18) so there are some issues with majority. Second, at 17 years old, I would have died of some sort of nervous hormonal condition within five minutes of meeting such a lady. I don’t think I was in any condition to talk to such a person until I was at least 19. Third, if the parentals are uptight about porn then this sort of thing will not go over very well.
…and iPods lead to piracy!!
Boys aren’t weird. Girls, are obviously the weirder gender, missy.
…they’ve got a Jesus doll that farts fire? gets his credit card out
I was going to suggest porn till I finished reading the OP.
Hey, it’s what 17 year old boys like. It’s the gift that encourages masturbation!
How about a handgun?
To all those suggesting porn/hookers, I’m curious:
Are you just trying to be brash or would you really give your little half-brother porn and hookers for his birthday?
I’m not trying to condemn you, as I was plenty fond of porn and hookers at 17 myself. I’m more thinking along the lines of how embarrassing it would be for me to give or receive anything like that from someone in my family. So would you really do it?