GRASS GIS (Geographic Resources Analysis Support System) is a Free Software Geographical Information System (GIS) with raster, topological vector, image processing, and graphics production functionality that operates on various platforms through a graphical user interface and shell in X-Windows.
I am planning to use GRASS as the core of the MapTribe software. This engine should provide a good basis for building the map of the city and to enable the landmarking of the urban tissue.