why volunteer in a combat zone when contractors will pay you so much money to do...anything. If you have any kind of degree or job skill you can probably get work as some kind of contractor, even a job with NGOs is going to pay well. I can't think of any other way you'd want to go over there as a US citizen right now. They'll put you up in the green zone and provide security.
If you just want to do humanitarian stuff you might consider Afghanistan. Even though it doesn't look like it on the news Kabul is actually a fairly safe city for Westerners. There might be more NGO jobs, or even some NGOs that will take volunteers. As I understand it a lot of time they wont just take volunteers because they don't want to deal with the liability and security.
Or...you could volunteer enlist in the US military... [;)]