I have an unpaid volunteer opportunity, non-teaching, with a humanitarian organisation in 2010 for a period of 6-9 months. I am a US passport holder. What would be the most efficient type of visa to get for this period? Since no wages are paid, is it necessary to obtain a work permit? If accommodations were provided by the host humanitarian organisation, would that constitute compensation from the perspective of a Thai work permit and change the nature of the most efficient type of visa to obtain?

While you very shortly will be getting from this forum some very sound advice, the answers to your questions are really best supplied by the organis(z)ation that is inviting you. If THEY do not know the answers, you might be in for a rough time.

You must have a work permit and to obtain that you must have a non immigrant visa and to obtain that you will likely require paperwork from the NGO. Pay or no pay is not an issue - work of any kind is.

Work Permit needed whether you get paid or not.

Yes, you need the work permit or you will be working (volunteering) illegaly. Also, without the work pemit you will not get an extension of stay.

Getting the work permit is not that difficult, but most organisations don't know how to go about it. Don't forget to bring your degree with you, some labour offices do want to see that for a work permit.