Kiva.org: matches entrepreneurs in poor countries with lenders in rich ones. You can choose whichever project/business you want to fund. For instance, this grocery owner in Kabul needs only $50 to complete a $1,075 loan to buy inventory for a year or this furniture maker who needs $350 to complete his loan. Little actions can build up quickly through sites like Kiva.
MicroPlace (another microloan site)
AGoodCause (which lets you raise money for charity while you shop)