OpenStreetMap is a wiki-like map that anyone in the world can edit.
At it's start OSM data was partial and inconsistent. But now (mid 2014)
we are seeing impressive coverage that can be used in GIS projects,
WEB maps and other Location Based solutions. OSM data is available
for free download from official OpenStreetMap
site or from this site:
in XML like format (.osm) or in ESRI Shape file format (.shp).
The huge advantage of OSM is it's open licence status and
use of crowd-sourcing for data collection. You can register and start
to edit spatial layers on OSM web site almost immediately: add roads,
house numbers, upload GPS tracks files, draw buildings, points of
interest and so on. You can contribute your data and fix/enrich edits
of other users.
Fragment of Milano city center - screen capture from OSM site. Pay
attention to high level of spatial layer details: shape of buildings,
pedestrian paths, gates, parking lots, POI names ...