Business Week published a slide show listing the "Greatest Givers":

Twenty-five philanthropists who made a difference, and changed the way we think about giving

We found in this list, well known philanthropists like Bill and Melinda Gates or Warren Buffet but also organization like kiva.org. There is also an item for the web site Charity Navigator:

 Charity Navigator, America’s premier independent charity evaluator, works to advance a more efficient and responsive philanthropic marketplace by evaluating the financial health of America’s largest charities.