United States Geological Survey
The USGS serves the Nation by providing reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.
I obtained a huge amount of data from the USGS. There are shape files available as well as web service access to various data layers. It takes a fair amount of time to search throughout the site, but some of the data is obscure and interesting at the same time.