Ushahidi ("testimony" in Swahili) is a free and open-source tool for information collection, visualization and interactive mapping.
Originally designed in the wake of the 2008 violence-marred Kenyan elections, Ushahidi has since grown to accommodate many disaster-response scenarios, including the Haiti earthquake and in various implementations throughout Latin America and Europe.
Click the link to visit Ushahidi's website and download the software.