Volunteering is a big part of American culture from my personal point of view. What kind of volunteering would you like to do? For example, if you like nature, you could consider working for the national parks. If you enjoy working with kids, consider working with the boyscouts or girlscouts. Here are some websites with more information.
http://www.volunteermatch.org/
http://www.serve.gov/
http://www.volunteer.gov/gov
http://www.nps.gov/getinvolved/volunteer.htm
http://scouting.org/Volunteer.aspx