why not try http://www.iamcambodia.org no fees and great experiences
-Moderator note: I think considering the topic (and the fact this is a long standing member) this link is legit even though the author has ties with the organisation-
[ Edit: Undid spam flag and added note. ]
I think that if you are looking for non-profit organizations its good to look to the country that you are going to.
When I went traveling I went with I-to-I and ended up paying a lot of money to them for the volunteer work that I was doing.
The people that I met on my travels, however, had organized their trips through organizations within the country they were traveling to and then had only had to pay for their flights and accommodation. Looking back I wish that I had done this as they all paid a lot less than I did, for doing essentially the same thing.
Where are you traveling to?
I did some work in Uganda at a school with IVI, its an aussie based group but it works with local aid agencies, depending on where you are and anyone can help. They do all sorts of things in all sorts of countries.
Not sure if it's what your looking for as they tend to be longer programs, and there is some cost involved but it is for use on the project. eg. my placement was $700AU but that was to cover food and building materials for the 3 weeks. (We slept at the school so no accomodation costs) and you just had to organise your own flights and visa.