If you're dubious about the claims made by travel companies promising 'authentic' tribal tours, then Tropic's trips are much more like the genuine article. The Ecuadorian operator has pioneered small-scale, sensitively-run trips to meet with remote nomadic hunter gatherer tribes in the Amazon, including the Huaorani (with whom it has helped set up a small ecolodge and 3,000-hectare nature reserve) and more recently with the Secoya People in the upper Amazon Basin.
Presented on the "Guardian Green Travel List 2010 - http://www.greentraveller.co.uk/content/green-travel-list-full-0