The Sierra Gorda Biosphere Reserve, located in the state of Querétaro, is the most ecodiverse Reserve in the country, and has been awarded UNESCO World Heritage status. Sierra Gorda Ecotours, developed by the Grupo Ecológico Sierra Gorda which oversees environmental conservation efforts, provides the opportunity for visitors to enjoy the biosphere while ensuring the conservation of biodiversity and contributing to the livelihoods of communities, who operate the three lodges. Sierra Gorda was the 2006 Destination Award Winner from the World Travel & Tourism Council
More info at SierraGorda.net

Spend two weeks playing with crocodiles on the Zambezi River. Spend the morning setting crocodile traps and spend the afternoon measuring and tagging and counting crocodiles. This is a beautiful area and the schedule allows plenty of time for birdwatching and fly-fishing.
14 days for $2849. at Earthwatch

Cross-Cultural Solutions is an international not-for-profit organization with no political or religious affiliations. It offers international volunteer programs in 12 countries, start dates year-round and programs from 1-12 weeks long. CCS operates in Brazil, China, Costa Rica, Ghana, Guatemala, India, Morocco, Peru, Russia, South Africa, Tanzania, and Thailand. CCS volunteers work in the areas of Social Services, Education and Health such as: Caring for infants & children, Teaching children, Teaching English, Assisting teachers, Observing and assisting local health professionals, Working with people affected by HIV/AIDS, Caring for the elderly, Caring for people with disabilities and Assisting teachers of Special Education. See the world while helping people, helps our planet, too.
Get all the info at Cross-Cultural Solutions

Travel out ot one of the most beautiful places in the USA, to Big Sur, California. Spend a week working with the US Forest Service improving eroded trails. This area is prone to fires and brush clearing may be added to the agenda.
Price: $395 includes all meals and camping at Divide Camp, an area located among mature pine and oak trees near the junction with the Pine Ridge and Carmel River Trails. April 28-May 5, 2007 More info at SierraClub.org

Work in Skaftafell National Park in southeast Iceland constructing and maintaining footpaths in the National Park. These paths help to protect the area’s fragile ecology from the impact from tourism. The work is strenuous, so this trip is great for the physically fit types. Accommodations - you will be sleeping in your own tent but hot showers and a base camp provide space for meals and conversation.

GB £480 ex flights - more details at Responsible Travel

The International World Wide Opportunities On Organic Farms Association (WWOOF) provides the venue for people around the world to learn first-hand about organic growing methods; enables both the organic farmers and volunteers to make organic farming a viable alternative; and, improves communications concerning the importance of organic farming in the world today.

Since its inception in 1971 London, WWOOF has grown to include participation of some 43 different countries including Australia, China, America and Finland.

Hosts listed through WWOOF are primarily pursuing a simple, sustainable lifestyle. Many are practicing Permaculture or Bio-dynamic growing methods. The farms vary from commercial to alternative co-operatives, or communities.

The Independents listed may include non-organic, yet are in keeping with the general ethic of respect for the environment; exchange of knowledge & expertise, food & accomodation in exchange for help on a voluntary basis.

All necessary information is available at the WWOOF Website: http://www.wwoof.org/