I had a very positive experience in my volunteer trip to Ecuador. I went with my girlfriend for 2 weeks into the heart of the rainforest, where we stayed at an animal rehabilitation center, which is sort of like a camp for grown-ups. Limited electricity, no hot water, bunk beds, etc. We were working directly with a diverse range of animals (monkeys, birds, tayras, kinkajous, to name a few). EcoVolunteer UP was the 'agency' that placed us, but the actual center is called Sacha Yacu. You can look them up directly, if you want.
Ecovolunteer Up is run by a nice woman named Maria. It's a small operation, run locally by her and a few other people. I was slightly reticent to commit to an organization with limited web presence and with substantially lower costs than the competition (like 80% less), and Maria refused to tell me the name of the center, which made me more uneasy. But I'm so glad I went for it, and trusted in this place, because it was a truly amazing experience. What's more, I discovered that there were several other volunteers at the center with us, who had been placed by much more expensive agencies. If you aren't familiar (I wasn't), the way these things work is you do all of your communication and payment through a company like EcoVolunteer UP or ProWorldVolunteers, etc. Some charge a lot more than others, and it's unclear what exactly you get for that price. In my experience at Sacha Yacu, there was literally no difference between the experiences of the different volunteers, but some paid a lot more than others to their respective agencies. So I would heartily recommend doing a lot of research and trying to find the exact program you want and the most reasonable agency that provides access to it. Then that agency will take care of your basic transportation, etc.
Again, the location is called Sacha Yacu. I understand why Maria wouldn't tell me the name. In theory you could circumvent the agency and go straight to the center, to save a few bucks, but I wouldn't recommend it. It's nice to have a middle-man, provided they don't charge you an obscene amount. And EcoVolunteer UP was very reasonable.
A few quick details about the experience at Sacha Yacu:
The volunteers seem to range from 19-30 years old. The family that owns/operates the place is very kind hearted and fosters a familial environment with the other volunteers. At any given time you might meet one or several members of the family. There are usually at least one or two 'long term' volunteers at the center -- people who are there for several weeks or even months, and subsequently develop some seniority there and can help run the place. The primary language that was spoken, at least in our experience was English. This was good because it bridged the gap for the various volunteers that came from all over the world. While we were there, we had about 10-12 people in the center at any given time, which was great. 9 or 10 volunteers and 2-3 members of the family. It's got a casual vibe, so there is no one there that would be called "staff." We would work about 6 hours/day, and then spend the rest of the time reading, talking, hiking, playing games, cooking, etc.
It was remarkably satisfying working closely with animals, as well as being outside all day, being active, meeting good people, and adapting to a completely new lifestyle. I lost some weight, got a nice tan, made friends, ate healthy, and disconnected from the rest of the world. I'll admit, at first I had a tough time with the fact that it's a physically challenging environment (lots of hiking, carrying, shoveling poop, etc.) and the sleeping situation isn't exactly luxurious, but the pros definitely outweigh the cons, and by the end of our stay there, I wasn't ready to leave.
So now you know everything you need to! Look it up! Sacha Yacu! It's great!