New, publicly downloadable GIS data sets are rapidly becoming available for socioeconomic, geological, ecological, and many other areas of land and “human geography” analysis. Below is a list of national GIS data relevant to park planning and public land use:

General
Open Street Map: Crowd-sourced data for the whole world, consisting of most things you'd find on a standard local paper map: points of interest, buildings, roads and road names, ferry routes etc. 

U.S. Cartographic Boundaries: Cartographic boundaries, including city boundaries, counties, school districts, voting districts, zip code areas and congressional districts. Mainly from the 2000 census, but some later data.

Land Cover 
USGS Land Cover Institute: Links to almost all land cover data sets.

Ecology
Protected Areas Database of the U.S.: Shapefile of all protected areas in the US including attribute data description uses, reasons for protection, and legal status.

Geology
Geological Maps of the U.S. States: Detailed geological maps with extended attribute data for each state.

Census
TIGER: U.S. Census data.
TIGER Line 2010: 2010 extracts from the census database. These files contain the boundaries of census areas, which can then be linked to census data tables.

History 
National Historical GIS: Census-extracted historical data between 1790 and 2010.