The U.S. government has some excellent open data sites where you can get geospatial and other data from all agencies. Individual agencies that produce useful geospatial data get their own boxes on the rest of this page.
The NGA "develops imagery and map-based intelligence solutions for U.S. national defense, homeland security and safety of navigation."
The USGS is one of the premier mapping agencies of the U.S. government. Since the late 1800s, they have produced topographic maps of the entire United States. Topographic maps often form the base for work in a GIS, and a digital topographic map can also be broken down into parts to form individual layers.
