For those who are interested in volunteering at a federal agency, it's always worth checking sites such as volunteer.gov. As an example, a keyword search for 'archeology' that I did this morning turned up offerings in Colorado, Utah, Pennsylvania, California, Hawaii, and Maine. You may also directly contact a specific park or agency, as there is often a volunteer coordinator who can let you know about current or upcoming volunteer opportunities and internships.

A few websites to check out (please add to this list if you know of others):

http://volunteer.gov/gov/
http://serve.gov/
http://www.nps.gov/getinvolved/volunteer.htm
http://www.passportintime.com/