The “crowdsourcing” project I got involved with was the Missing Maps project, part of OpenStreetMap. OpenStreetMap is intended to create a freely available and freely editable (like Wikipedia) mapping project. One of its goals is to map parts of the developing world that aren’t economically worth bothering with for firms like Google; organizations like Médecins sans frontières rely on OSM maps when they go places like remote parts of the Congo. One of the components they’re currently soliciting help with is in mapping part of the coastline of Lake Tanganyika in eastern DR Congo to help the operations of MSF and other relief NGOs.
Basically how it works is this: You “check out” a map grid square – locking it so nobody else stomps on your edits – and trace objects visible on Bing satellite images, tagging them as appropriate. The element I worked on was finding all the little clusters of dwellings scattered around the countryside in a grid square just inland from the lake shore, tracing their perimeters to form GIS polygons, and tagging them as residential areas.