M.M.
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.