MicroFinance is the principle of teaching a man to fish instead of giving them a fish. These are organizations that give small loan amounts that traditional banks would consider too small. These are loan to help people with no capital get the small seed money they need to start a business and become self sufficient.
Kiva is the absolute most impressive site I have come across today for social entrepreneurship. Kiva has thousands of profiles of people that need micro-loans all over the world. You log in and can give as little as $25 towards someone's loan. They have a 98.93% repayment rate as of January 2012.
Accion is the largest micro lending institution in America with branches all over the country. The CEO of Accion Texas was recently asked to be on President Barack Obama's Council on Financial Capability. I have worked with Accion several times and they are masters of their craft.
Grameen America is the US division of Grameen Bank which originated in Bangladesh. The founder, Muhammad Yunus is the actual creator of micro-finance and was awarded the Nobel Peach Prize in 2006 for his work in helping the underprivileged. You can read about his amazing story in his book, Banker to the Poor.