There are a few organizations that accept donations to save land in the South American rainforests:
Rainforest Action Network's Protect-An-Acre works to protect the rainforest and the people who live in it. As opposed to traditional "buy-an-acre" programs, the RAN program supports community organizing, alternative economic initiatives and resistance to logging and large-scale development in rainforests.
The Tropical Rainforest Coalition operates the Save-an-Acre program, which ensures that acres are incorporated into land preserves. The coalition accepts donations from individuals and groups. A tax-deductible donation of $50 will save one acre of rainforest.