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At 18 my son got his computer. He also got his first phone.
Kindle
iPod
iPad
GC for iTunes
tv for his room, my kids will be really upset if they know I just recommended that one.
Car gadgets
Tom Tom
tools
Wii or other game counsel
compact dvd player
Game, like Dungeons and Dragons (ugh) or such
Dvd's of a favorite tv show. My little guy's is M*A*S*H
NFL gear, like a Bears jersey :o)
Tickets to a game NFL/NHL/NBA
Compound Bow
BB Gun
new Bicycle
Weights and weight bench

Mine is 22. These are some of the things he's gotten as gifts or for himself.

Does he like music? Does he use iTunes? Would he like a Kindle? Does he drive? Would he like any car accessories, maybe for his MP3 player or phone? Does he drive a lot? A Garmin or TomTom? Or would he like a night out? We rented a pool hall (well part of one, and a nice one, not a smokey hole in the wall) and got food for my SS's 17th birthday and he had a rotating group of friends come by for the 3 hours. Or maybe take him out to his favorite place? My SS likes to eat and is always up for a good meal.

Movie tickets. Restaurant gift cards. iTunes gift cards. Gas cards if he has a car.

If he's getting an apartment or going to college, get him stuff for the apartment or college-themed shirts, shorts, etc.

That's what my son loves!

Depending in your budget: gift cards to his favorite places. If he goes to the movies, same thing. A small party at a place like Dave and Busters for him and his friends would probably be a blast. Money, of course, will always work :)

Is he going off to college or anything? If so, you can get him something for his dorm room. Otherwise, AV gave some good ideas with the kindle/itunes.... In our family, going out to dinner is always a nice gift.

Gift cards to fast food restaurants. I randomly send them to my 17 and 22 yr old sons and they love them. I buy in $10 or $20 increments. It lets them eat while out with friends.

My husband offered to give his little brother $100 to invest. I guess it was supposed to be a gift and teach him responsibility at the same time. (He never did take us up on the offer though.)

How about funding a special adventure for he and a friend - maybe a fishing trip or tickets to a sporting event he would enjoy?? How about a year long membership to something he has an interest in (gym, magazine subscription, museum, etc etc)