Ethical, sustainable, ecotourism (or whatever you want to call it), is a different way of travelling. It's intended to benefit or at least take into account the effects of tourism on local peoples by respecting their environment or donating a percentage of the profits to development projects.
There are hundreds of such agencies, charities and organizations out there catering for anyone from school-aged children to gap-year travellers to adults. With organizations such as Raleigh International, you can even get actively involved in building and development in underprivileged areas of the world whilst you discover the country, its culture and people.
Websites to help you get started: