Some links that help to reinforce the idea of small actions combining to pay big dividends:

http://www.freerice.com – learn new words and earn grains of rice to be donated
http://www.freepoverty.com/ – geography game turns into donations of water
http://jogogreen.com/ – 10 right questions = a planted tree (currently offline)
http://www.goodsearch.com/ – give to charity just by searching the internet
http://www.freekibble.com/ – questions = food for animal shelters
http://www.therainforestsite.com/clickToGive/home.faces?siteId=4 – clicks equal saved rainforest area

and another new one:

http://www.kiva.org/ - make a loan to an entrepreneur across the globe for as little as $25, to alleviate poverty (I think this would be a neat class activity)