There are two purchasing programs that The Rainforest Challenge uses.

One, The Children's Rainforest in Costa Rica, is a well-established park founded by the efforts of school children. You can purchase acres through this program for $100.

The other program is The Rainforest Preservation Program in Brazil. The acres purchased are added to a preserve which is monitored by indigenous peoples who are supported partially by your donation. You can purchase acres through this program for $25.

There are other preservation and "Spare an Acre" Programs out there. Most noticably The Rainforest Relief's program, where through a donation to their organization, they can launch campaigns to halt the use of tropical hardwoods, thus reducing demand for tropical timber. For information about their program, check out their web site.