I came across Volunteer South America.net and reviewed it here about a month ago about one month ago. The site is a great resource for potential volunteers, offering links to organizations which need volunteers and charge either no fee or a low one to cover basic expenses. A few samples:

• Children International Ecuador (note that the pages is internally hosted page by Volunteer South America; founder Steve McElhinney offers this as a free service to organizations in South America which would like volunteers but don’t have their own website). You can teach English or computer classes, assist in the health clinic or library, be a youth tutor - in cooperation with teachers working with young people with learning disabilities, get involved in small business advising for locals looking to better their lives, or – if you’re a doctor or psychologist – participate in those branches of Children International’s work.
  
  Volunteering currently takes place in Quito, and while volunteers don’t pay a fee, they are responsible for their own accommodation, food and transport which Children International estimates at about $380 per month (obviously excluding travel to and from Ecuador). Note that volunteers need to show proof that they have no criminal record – find some information on this document based on my own experience of obtaining one for teaching in Europe here.
• Ak’ Tenamit, a non-profit Mayan-run organization, works in the rain forests of eastern Guatemala and “provides the area with education, healthcare and community development, while supporting the sustainable use of national park resources.” Volunteers generally need some background in medicine (nurse, doctor or final year med student); the time commitment varies from three months for a clinic volunteer to one year for a more involved role. Free room and board is provided, but volunteers must pay for their own way to Guatemala.
• It appears that La Montana in Argentina is primarily a Spanish language school, but they also have information about participating in volunteer projects; volunteers who have spent four weeks in a language program pay a $100 volunteer fee, and those interested only in the volunteering portion pay $125. I didn’t find any information about other expenses. They mention possibilities to volunteer in different areas – social, education, environment, though it likely depends on the local organizations and what’s going on at the moment.

For a wealth of links to South America volunteer opportunities, as well as links to other sites aimed at volunteers in general, visit Volunteer South America.net.