Since 1976, Habitat for Humanity has been working with people around the globe to fulfill their vision of "a world where everyone has a decent place to live". They do this because the need for affordable housing exists in every community.
Habitat works in over 30 countries with their Global Village program and since their founding, they've helped more than 29 million people build or improve the place they call home. Families apply for Habitat homes that will be sold to them at no profit. Once approved, they are required to make a down payment and monthly mortgage payments. In addition, they must put in 500 hours of "sweat equity" into the building of their own home and the homes of other Habitat partner families. Habitat considers the work they do a hand up, not a hand out.
I've volunteered on seven Habitat Global Village trips (Guatemala, Peru, Romania, India, Chile, Madagascar and Kyrgyzstan) and they've all been fabulous experiences. Being on a team of volunteers who are using vacation time to build houses with the families that are going to live in them is inspiring. Getting to interact with local people you might not meet as a tourist is an added bonus.