The Philosophy of the Poverty Action Lab: They are conducting RCTs to quantify the impact of social interventions on the extreme poor. This is inspiring because they have proven that it is possible to make very significant progress against the biggest problem in the world through the accumulation of a set of small interventions.
What I think would be the most emotionally gratifying engineering project (at least in theory): Integrated Rural Development
The Millennium Villages Project takes a holistic, community-led approach to sustainable development by improving access to clean water, sanitation and other essential infrastructure such as education, food production, basic health care, and by focusing on environmental sustainability, Millennium Villages claims to ensure that communities living in extreme poverty have a real, sustainable opportunity to lift themselves out of the poverty trap. Their work unites science, business, civil society and government in these efforts by empowering communities and partners.