Fast food, bussing tables at a restaurant, etc are all possibilities.
If you like kids there's always babysitting.
And if you are good with people, you can offer to give computer lessons on basic things (installing programs, using word/excel/powerpoint) to the elderly or people who didn't grow up around computers.
Babysitting and/or offering computer tutoring would be good since they'd be "under the table" jobs that wouldn't generate IRS paperwork. Many companies are hesitant to hire below 18 for any decent jobs due to hourly limitations. I know when I was a manager for Borders we did not hire below anyone below 18 because it didn't do us a lot of good to have someone who could work from, say, 5pm-9pm if we needed people from 5pm-10pm (and depending on your state there are severe penalties for letting minors work past certain hours.)
Fast food generally doesn't have the same issue. I know we hired below 18 at Panera, though technically you couldn't use certain equipment if you were a minor. Apparently turning 18 makes you magically capable of using industrial bread slicers, while being 17 and 364 days makes you an idiot. 🙂