Volunteer opportunities in the United States?

[Crystal]

A friend and I live in Michigan, and we do a lot of volunteer work around our separate hometowns. However, before we go off to college, we wanted to do a little bit of traveling and volunteering somewhere different, somewhere new. We couldn't go abroad, so we've been trying to find places that help place volunteers around the U.S. If you know of any, it would be awesome if you could tell us and give us links! Thanks a lot!