Data provided by the National Center of Atmospheric Research's GIS Initiative. Geodata for the US and global vector data.
A couple of unique datasets from the Chicago, historical and present. Sprinkled with a couple good links to data clearinghouses.
User-driven content.
Search and download geodata like street level and ortho-aerial photography.
Feeling ironically lost in a world of maps? These critical resources will help you get your bearings!