...
Census TIGER Shapefiles | TIGER Line files for legal and statistical areas in shapefile format, updated annually |
US Census Cartographic Boundary Files | Generalized extracts of the bureau's TIGER Line files for several administrative, legal, and census areas in the US |
National Historical Geographic Information System | Historical census boundaries and data for the US from 1790 to 2010present. New users must register, but there is no cost and downloads are free |
National Land Cover Database | Land use and land cover data for the entire US |
USGS National Map Viewer | Download imagery, elevation data, land cover, and some boundary files via the USGS' web mapping interface |
PRISM Climate Group | Daily, monthly, and annual climate rasters with temperature and precipitation data for the US |
Libre Map Project | Topographic Historic topographic maps (DRGs) for the entire US |
...