Accoustic Outreach
http://www.acousticoutreach.org/kidscare/
Lists a bunch of short descriptions of kids who started programs that make a difference.
Kids Care Clubs
http://www.kidscare.org/kidscare/index.htm
Clubs and resources for kids to volunteer.
IdeaList
http://www.idealist.org/kt/youthorgs.html
This page has links and explanations of good non-profits started by kids
Are You Into It?
Kids volunteering, related to 4-H
ServeNET
A good resource to find out about non-profit organizations
GuideStar
Very comprehensive and detailed database of non-profits.