Irrespective of whether you do voluntary work or get some form of sponsorship, you'll need a visa anyway. Entry under the Visa Waiver Program is only possible for stays of 90 days or less.
You'll have to decide which is the applicable type of visa from the list down the left of this page:
http://www.usembassy.org.uk/cons_new/visa/niv/ index.html
(A tourist visa would probably be OK for voluntary work but I recommend contacting the US embassy to check. The contact details are here:
http://www.usembassy.org.uk/ukaddres.html ).
Chris