The Map Kibera project is an awesome example of community-powered mapping succeeding in a situation where commercial mapping providers would never venture: Kibera, the largest slum of Nairobi, Kenya with a population estimated at near a million. The project will train local residents to create community maps - using OpenStreetMap tools and techniques - and encourage the use of community-generated maps in relief and development efforts.
WaterGoodness is a new site allowing users to share and find water quality reports across the United States. The site combines water quality and pollution data from a variety of government sources with visitor reports of their local water quality.
By searching the US Environmental Protection Agency's database, GreenSpaceMap offers visitors the ability to locate environmental threats in their neighborhood or region. Locations like Superfund sites and brownfields are visible once you type in a starting address. This is also an entry in the Sunlight Labs Apps for America contest.