Today I am volunteering with IntiWawa (http://www.intiwawa.org/index.aspx) in Arequipa since a week and it's been very nice.
I mostly do the "Jardín" which is a kind of Kindergarten. The kids are between 3- and 5 years old and very sweet.
The kids are from a very poor background.
They come to the Jardín to learn basic things to get a better education and better chances for their lives.
At the end of the day they get some food which is prepared by one of the kids' mothers. Every someone else's mother.
There a more projects to do, like "Tareas" which is helping the kids with their homework.
Just check their website in case you want to know more about it or if you find some energy to get involved.
The work itself is free (as it should be for voluntary work, but most organisation charge you a fortune for it), you just pay for your accomodation and food which is quite cheap.
So, if you plan to travel South America, consider to stop by and help for some weeks.