Online GIS Data Sources

Spatial Data Portals & Clearinghouses
Boston Area Data
Massachusetts Data
New England States GIS
U.S. Agencies
International Agencies
Satellite Imagery
GISTA Data by Topic

Spatial Data Portals & Clearinghouses

ArcGIS Online - ArcGIS data server - includes a number of world, US, and US state data sets that can be brought up directly in ArcGIS (click on a data set and if you have ArcGIS, it will open the program; alternatively, in ArcGIS, click on File - Add Data From ArcGIS Online)
EROS Data Center - Great source for U.S. topographic data and internatonal DEMs.
David Rumsey Historical Map Collection - Digital, georeferenced historical maps
GIS Data Depot
Geography Network
Geospatial One Stop
Global Spatial Data Infrastructure Association (GSDI) - List of portals by scale
FGDC Clearinghouse Registry - Registry of data portals.
Harvard Geospatial Library - Free Digital Chart of the World (DCW) data and historic maps.
Starting the Hunt: Guide to Free Geospatial Data - Great site for state-wide data.
US National Atlas
MapAbility.com - Free VMAP1 international data.

Boston Area GIS Data

Boston Atlas
Boston Redevelopment Authority
Brookline GIS
Cambridge GIS

Massachusetts GIS Data

MassGIS - State GIS Office - Great site for Massachusetts.
ArcGIS Online - Massachusetts - MassGIS publishes its data on ArcGIS Online - once on the site, scroll to find Massachusetts and click on the map - it will open in ArcGIS if you have it installed on your computer.
Mass Dept. of Fish & Game
Mass Dept. of Public Health

New England States GIS

Starting the Hunt: Guide to Free Geospatial Data - Great site for state-wide data.
Connecticut Map and Geographic Information Center (MAGIC)
Connecticut Department of Environmental Protection GIS Data
Maine Office of GIS
New Hampshire GRANIT
New York State GIS Clearinghouse
Pennsylvania Spatial Data Access (PASDA)
Rhode Island GIS
Vermont Center for Geographic Information, Inc. (VCGI)
ArcGIS Online - States - MassGIS, New Hampshire, Vermont, Maine, and many other states publishe their data on ArcGIS Online - once on the site, scroll to find the state of interest and click on the map - it will open in ArcGIS if you have it installed on your computer.

U.S. Agencies

Bureau of Transportation Statistics Atlas Data
CIA World Factbook - International maps and statistics
Federal Emergency Management Agency (FEMA)
Famine Early Warning System - Livelihood analysis and mapping.
GEONet Names Server (GNS) - Get coordinates from NGA's international place name server.
NASA Land Processed Distributed Active Archive Center
National Center for Health Statistics
National Hisotric GIS - Historic census geography provided by U of Minnesota.
National Geodetic Survey
National Geospatial Intelligence Agency (NGA)
NGA Raster Roam Engine\ - Good for international data
National Institute of Justice Mapping & Analysis for Public Safety
National Oceanic and Atmospheric Administration (NOAA)
NOAA National Environmental, Satellite, Data, and Information Service
NRCS Soil Survey
US Census Bureau
US Environmental Protection Agency (EPA)
US Fish & Wildlife
USDA Geospatial Gateway
USGS EROS Data Center - Great source for U.S. topographic data.
USGS GEO-Data Explorer (GEODE)
USGS Node of the National Geospatial Data Clearinghouse
USGS Water Resources NSDI Node

International Agencies

World Heritage Centre - Index of National Mapping Agencies
Index of International Statistical Agencies
Census Dates for Countries and Areas of the World: 1945 to 2014
Afghanistan Information Management Service
Africa Data Dissemination Service
Australian Consortium for the Asian Spatial Information and Analysis Network (ACASIAN)
Center for International Earth Science Information Network (CIESIN)
EastView Cartographic - Commercial vendor, great source of international topo maps and data.
Famine Early Warning System (FEWS Net) - Livelihood maps and GIS data
FAO Geoportal
GEOBIS - Commercial vendor specializing in international GIS and statistical data.
Geographic Information Support Team (GIST) - Great source of international admistrative geography.
Gfk GeoMarketing - Commercial vendor, great source of international administrative geography.
Go-Geo UK GIS
HumanitarianInfo.org - Index of Humanitarian Information Centers
MesoStor - Mesoamerica's free data delivery system.
NatureServe - World-wide animal data for download.
OCHA GeoNetwork - FAO, WFP, and UNEP shared database of GIS data, maps, statistics, etc.
Poverty Mapping - a global spatial database of poverty mapping examples and possible indicators.
Southern Africa Human-development Information Management System (SAHIMS)
United Nations Environment Programme (UNEP) - Global & Regional Integrated Data Centres
UNEP GEO Data Portal
United Nations Statistics Division
World Wildlife Foundation (WWF) Data Download - International ecological and hydrological data.

Satellite Imagery

University of Maryland Global Landcover Facility - Open source for Landsat TM and ETM data
Landsat
SPOT Image
GeoEye
Digital Globe
MDA Federal, Inc.