According to a recent survey, the most generous givers to charity by percentage of income are those making less than $20,000 a year. They are nearly 2% more generous than everyone else except those making over $100,000 per year, whom they still beat by 1.5%.
Surveys of charitable giving have always been a little suspect, so I'm not sure how much to make of this. Conservatives, for example, have long crowed over polls showing right-wing voters are more generous, as proof apparently that "ZOMG who needs taxes!!!" However, it's not quite clear how much of those donations are going to religious institutions, which are not always, or even often, the model of selfless charity. Likewise, the supposed generosity of the rich is often touted based on the total amount of money given but that, to no one's great surprise, appears to be overstated.