Stanford GeoPortal
A listing of websites offering free GIS data for download. Arranged by geographic area and sub-grouped by theme (e.g., biota, climatology, energy, environment, health, society, transportation, etc.). Curated by Stanford University Libraries.
OpenStreetMap.org
OpenStreetMap is built by a community of mappers that contribute and maintain data about roads, trails, cafés, railway stations, and much more, all over the world. Help for using the data and exporting data from the database can be found in the site wiki.