I do a iTunes gift card of a lessor value then get a tshirt - I get cool tshirts online so they're of alternative pop culture...except sometimes sizing can be odd, I've yet to see one fail. I just hate the gift card/money thing (I know they love it but they'll grow up to think you just go - happy birthday honey, and had over cash out of their wallet....)
I think you can't go wrong with cash and itunes gift cards. It's so hard to know what they like, maybe gift cards for video games stores, electronic stores too? but then again...they're still gift cards.
It depends on the boy. If they really won't tell you what they want and you don't have a clue I would probably go with a gift card. Better that than something they really don't want or that is even a bit offensive. Don't give a teenage boy aftershave as a passive aggressive hint that they should shave and fellow adults of the world, please, please don't give teenagers pimple cream as a present!
Maybe an experience gift, like concert tickets would be good if you knew what sort of music they liked.