There are a couple of great volunteering opportunities in Cambodia worthy of a mention.
Habitat for Humanity
Sunshine Foundation (orphanage)
I did a house build with Tabitha last year and it was one of the most humbling experiences ever. You do need to fundraise beforehand, but what I loved about this organisation was that 100% of funds rasied go towards the building of a home. The recipients also have to save some money as well so that its not a "gift" as such, and they participate in a microsavings loan scheme as well as vocational training. It can take up to 7years to save for a house (USD$25).
I'm a little put off by volunteer tourism as there are some really unscrupulous organisations out there making a hefty profit on those in most need, as well as kind hearted volunteers.
Its great to see you doing some research beforehand. about 6 years ago