Individual states maintain online "clearinghouses" providing access to geospatial data resources pertaining to their state.
ESRI's ArcGIS Resources site provides a variety of resources for use in developing ArcGIS models. Examples include:
Atlas of the Biosphere : Center for Sustainability and the Global Environment (SAGE)
"The idea behind this website is simple: gather as much information about the environment as possible, and deliver it to as many people as possible. We are dedicated to bringing environmental information to the widest possible audience. Although we specifically target high school to university students and teachers, anyone can benefit from a stroll through our collection".
Atlas of Historical County Boundaries
The Atlas of Historical County Boundaries is meant to be a resource for people seeking records of past events, and people trying to analyze, interpret and display county-based historical data like returns of elections and censuses, and for people working on state and local history projects. The special interests of those potential users range from history to demography, economics, genealogy, geography, law, and politics. While many of these goals can be achieved using the Atlas' Interactive Maps, the downloadable data can be used with various GIS (Geographic Information Systems) programs to create specialized projects.
Census Bureau TIGER and TIGER/Line Mapping Center
Topologically Integrated Geographic Encoding and Referencing (TIGER) product information from the US Census Bureau, including up-to-date U.S. Census boundary shapefiles and relationship files.
Census Bureau "On The Map" - Employment and Worker Statistics/Maps
OnTheMap Version 6 is the sixth generation of OnTheMap, a web-based mapping and reporting application that shows where workers are employed and where they live. It also provides companion reports on age, earnings, industry distributions, race, ethnicity, educational attainment, and sex.
Cropland Data Layer (USDA)
The Spatial Analysis Reseach Section released the 2009 Cropland Data Layer (CDL) for ALL Continental US States. All state products were issued in unsmoothed format, with the exception of Florida (i.e., citrus smoothing applied) with associated metadata. The file sizes are quite large and range between 13 - 100 megabytes per download.
DATA.GOV (Tabular and Statistical data from federal agencies)
Economic Data Resources : USDA Economic Research Service (ERS)
ERS's highly trained economists and social scientists conduct research, analyze food and commodity markets, produce policy studies, and develop economic and statistical indicators. The agency’s research program is aimed at the information needs of USDA, other public policy officials, and the research community. ERS information and analysis is also used by the media, trade associations, public interest groups, and the general public.
ELEVATION DATA: United States Interagency Elevation Inventory
The U.S. Interagency Elevation Inventory displays high-accuracy topographic and bathymetric data for the United States and its territories. The project is a collaborative effort between NOAA and the U.S. Geological Survey, with contributions from the Federal Emergency Management Agency. This resource is a comprehensive, nationwide listing of known high-accuracy topographic data, including lidar and IfSAR, and bathymetric data, including NOAA hydrographic surveys, multibeam data, and bathymetric lidar. This inventory was completed May 2012 and will be updated annually.
The information provided for each elevation dataset includes many attributes such as vertical accuracy, point spacing, and date of collection. A direct link to access the data or information about the contact organization is also available through the inventory.
EPA Smart Location Database: A National Dataset for Characterizing Accessibility and the Built Environment at the Neighborhood Scale
A large body of research has demonstrated that land use and the built environment can have a significant effect on travel behavior. An EPA-funded meta-analysis of this literature, "Travel and the Built Environment," summarized the measurable effects of built environment variables on residents’ travel behavior. It found that driving is most strongly related to measures of destination accessibility and street network design, while walking is most strongly related to measures of land use diversity, intersection density, and the number of destinations within walking distance.
These findings can help to inform travel demand studies as well as land use scenario impact analysis. However developing data about these built environment characteristics can be expensive and time consuming. EPA’s Smart Location Database makes such data more widely accessible.
FEMA Map Service Center
Flood Rate Insurance Maps [FIRMs] on the Web
Forest Service Geospatial Clearinghouse (USFS)
Raster and vector forest data, state and private forestry assessment data, maps, etc.
Since 2005 Geodata.gov has provided the largest web-based access at a single-point for maps, government data, and geospatial services—the Geospatial One Stop. Data.gov, launched in May 2009, provides access to over 400,000 (primarily geospatial) datasets from 172 agencies across the Federal government. On October 1, 2011, Geodata.gov moved to Geo.Data.gov, within the Data.gov infrastructure.
Geospatial Data Gateway
USDA Natural Resources Conservation Service. This site includes county and state level data, including but not limited to: DRGs, DEMs, census data, land cover, quad indexes, digital orthophotos, etc.
The GIS Data Depot
(GeoCommunity Data Clearinghouse) Available data include DOQQs, DRGs, DEMs, Census data, hydrology, land use/land cover, etc.
The National Map
(USGS) Provides public access to high-quality, geospatial data and information from multiple partners to help support decision making by resource managers and the public. The National Map is the product of a consortium of Federal, State, and local partners who provide geospatial data to enhance America's ability to access, integrate, and apply geospatial data at global, national, and local scales.
The National Map "Streamer"
Streamer is a new way to visualize and understand water flow across America. With Streamer you can explore our Nation's major streams by tracing upstream to their source or downstream to where they empty. In addition to making maps, Streamer creates reports about your stream traces and the places they pass through.
The National Spatial Data Infrastructure Clearinghouse
(NSDI) - Federal Geographic Data Committee (FGDC)
The National Wetlands Inventory
Digital wetlands data from the U.S. Fish and Wildlife Service, Branch of Habitat Assessment.
Starting the Hunt: Guide to Mostly On-Line and Mostly Free U.S. Geospatial and Attribute Data
Guide to finding free sources for U.S. geospatial data (from the University of Arkansas)
EarthExplorer: USGS Geographic Data Download
Quick links to many of the popular USGS raster datasets for direct downloading including Digital Elevation Models (DEMs), National Elevation Dataset (NED), National Land Use & Land Cover (LULC), and the National hydrology Dataset (NHD).
USGS Digital Topographic Map Download
Download free USGS topographic maps at various scales. Use the "Map Locator" to select a site, view the quad boundaries and download a PDF.
Antarctic Digital Database
The SCAR Antarctic Digital Database (ADD) is a seamless compilation of topographic data for the continent of Antarctica to 60°S. The SCAR ADD consists of geographic information layers including coastline, ice-shelf grounding line, rock outcrop, elevation data and human-presence features such as Research Station locations.
CARPE - Central African Regional Program for the Environment
An interactive web GIS for the Congo Basin and the CARPE Data Explorer lets you search and browse CARPE-related data and metadata. You can search by defining an area of interest, or by specifying a content type and an optional keyword.
China Historical GIS
The China Historical Geographic Information System, CHGIS, project was launched in January 2001 to establish a database of populated places and historical administrative units for the period of Chinese history between 222 BCE and 1911 CE.
CIESIN - Center for International Earth Science Information Network
CIESIN works at the intersection of the social, natural, and information sciences, and specializes in on-line data and information management, spatial data integration and training, and interdisciplinary research related to human interactions in the environment.
Famine Early Warning Systems Network
The U.S. Agency for International Development (USAID) Famine Early Warning Systems Network (FEWS NET) is an information system designed to identify problems in the food supply system that potentially lead to famine or other food-insecure conditions in sub-Saharan Africa, Afghanistan, Central America, and Haiti.
FAO - GIS Gateway
GIS gateway to geosaptial resources within the Food and Agricultural Organization of the United Nations.
Access the FAO GeoNetwork site to search and download spatial data.
France: Open Access to French Government Data
Beta version as of December 2011. Open Access Portal to French Government inforamtion.
GeoCommons: Open Analysis, Data, and Maps.
GeoCommons is the public community of GeoIQ users who are building an open repository of data and maps for the world.
GeoGratis - GIS data for Canada
GeoGratis is a portal provided by the Earth Sciences Sector (ESS) of Natural Resources Canada (NRCan) which provides geospatial data at no cost via your Web browser.
Free GIS data downloads from the Australian government.
Global Land Cover Facility
The GLCF is a center for land cover science with a focus on research using remotely sensed satellite data and products to assess land cover change for local to global systems. Direct download global land cover data for use in a GIS.
Go-Geo! - GIS Database for the United Kingdom
Go-Geo! is a tool designed to help you find details about geospatial datasets and resources within Great Britain.
Map Library.org - Detailed World Administrative Boundaries - From the Mapmaker Trust, Scotland.
Detailed second-level administrative boundary data for countries of the world. Check data sources for authenticity.
Natural Earth is a public domain map dataset available at 1:10m, 1:50m, and 1:110 million scales.
Oak Ridge National Labs Distributed Active Archive Center (DAAC)
Biogeochemical and ecological data useful for studying environmental processes. The extent of data ranges from site-specific to global, and durations range from days to years.)
SAFARI - Southern Africa Regional Science Initiative
Direct access (via FTP) to soils, hydrology, climate, satellite imagery,vegetation and wetlands data for the southern Africa region.
SALB - Second Administrative Level Boundaries
The Second Administrative Level Boundaries (SALB) dataset is a dataset aiming at improving the availability of information about administrative boundaries down to the second subnational level.
SEDAC - Socioeconomic Data and Applications Center
SEDAC, the Socioeconomic Data and Applications Center, is one of the Distributed Active Archive Centers (DAACs) in the Earth Observing System Data and Information System (EOSDIS) of the U.S. National Aeronautics and Space Administration.
International streets data and transportation networks from OpenStreetMap data.
WorldClim - Global Climate Data
WorldClim is a set of global climate layers (climate grids) with a spatial resolution of about 1 square kilometer.