Whether you're looking for summer work for yourself or are trying to help a friend, here are a few good places to start looking:
The American Camp Association hosts a job board for camps across the country.
Cool Works: Jobs in Great Places specializes in postings for seasonal positions at national parks and resorts.
Groove Job specializes in part-time and temporary positions. Check-out their information page for teen job seekers. http://www.groovejob.com/jobs/teen-jobs/
New York City Department of Parks and Recreation: find a cool internship or look into becoming a "Whistle Worthy" lifeguard (their words, not mine.)
New York City Department of Youth and Community Development Jobs & Internships section http://www.nyc.gov/html/dycd/html/jobs/jobs_internships.shtml
Student Jobs for the Federal Government: Take advantage of some of the best paid internships available and get your foot in the door with Uncle Sam. Positions are available for students at all levels including high school.
Snag-a-Job is a job site designed with the part-time/temporary worker in mind. They also have a section dedicated to teen job seekers
Keep in mind that if you are a minor, special restrictions apply regarding the number of hours you work and possibly the shifts you work. For more information about relevant labor laws as well other information to help you in your job search, take a look at About.com's guide.
College students take note: Summer is a great time to take advantage of the outstanding internship opportunities companies offer. To learn more about top internships, visit the Science, Industry and Business Library and take a look at Vault's Guide to Top Internships, 2010.