This book introduces us to a new breed of world changers, the "social entrepreneur".
There was an excellent series about this on PBS, "The New Heroes" which was hosted by Robert Redford.
Ashoka and the Skoll Foundation are also great places to learn more about the people who are doing it.
While it seems that many folks are pessimistic about the world today, I think there is a lot more to be optimistic about that than they realize.