Spatial Data on the web, worldwide: Geographic Information Systems (GIS) Lab at MIT

OpenStreetMap: "OpenStreetMap creates and provides free geographic data such as street maps to anyone who wants them. The project was started because most maps you think of as free actually have legal or technical restrictions on their use, holding back people from using them in creative, productive, or unexpected ways."

Download openstreetmap data in different formats: http://downloads.cloudmade.com/

GeoCommons - Finder! lets one search for data, Upload, download, and convert between Shapefile, CSV, and KML. Maker! enables one to make maps on the web.

Aster global digital elevation model (30 meter resolution for the globe):
https://wist.echo.nasa.gov/~wist/api/imswelcome and http://www.gdem.aster.ersdac.or.jp
More about this dataset at: http://www.nasa.gov/topics/earth/features/aster-20090629.html

Shuttle Radar Topography Mission (SRTM) (90 meter resolution outside USA, 30 meter resolution in USA):

http://seamless.usgs.gov/
http://srtm.csi.cgiar.org/
http://www2.jpl.nasa.gov/srtm/

GTOPO30 & Hydro1k - a free global digital elevation model (DEM) with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer). HYDRO1k is a geographic database developed to provide comprehensive and consistent global coverage of topographically derived data sets, including streams, drainage basins and ancillary layers derived from the USGS' 30 arc-second digital elevation model of the world (GTOPO30).

Global Land Cover Facility (GLCF)
"...the best open source for Landsat TM and Landsat ETM on the planet..." - Digital Terrain Modeling and Mapping Journal
In addition to Landsat imagery, GLCF is hosts MODIS, AVHRR, and more.

United Nations Environmental Program - This online database holds more than 400 different variables, as national, subregional, regional and global statistics or as geospatial data sets (maps), covering themes like Freshwater, Population, Forests, Emissions, Climate, Disasters, Health and GDP.

The Geography Network provides access to a large collection of geographic data from many of the world's leading data publishers. The data collection includes a number of online data sets that you may preview online and download immediately or bring directly into an ArcMap project. The collection also includes links to many other data sets that you may acquire directly from the data publisher.

GNS WMS service
GNS WFS service

Geoscience Data Link: The Geoscience Data Link is hosted by the Lamont-Doherty Earth Observatory. Visitors can search by data types, date, and regions. Includes information on marine, continental and polar environments. Includes a global bathymetry database, which allows users to look at maps and download grids.

NCAR Scientific Support Section : A large number of datasets (some free and some for a fee) from the National Center for Atmospheric Research.

Digital Chart of the World Web Server: The Digital Chart of the World (DCW) is an Environmental Systems Research Institute, Inc. (ESRI) product originally developed for the US Defense Mapping Agency (DMA) using DMA data. We used the DCW 1993 version at 1:1,000,000 scale.The DMA data sources are aeronautical charts, which emphasize landmarks important from flying altitudes. This explains why there is a separate aeronautical theme with all conceivable airports, yet why on some themes small islands and lakes are simply unnamed points.

The Harvard Geospatial Library provides an online tool for locating data around the world. Dated located on their web site includes; Digital Chart of the World, Census 2000 Block Groups and Tracts, ESRI Data & Maps (6 CDs), MassGIS vector data, municipal data for Boston, Cambridge, Brookline, and the National Transportation Atlas.

The Center for Advanced Spatial Technologies (CAST) is a great site to start searching for data. The site includes a listing of data by 28 different subject areas including; crime, census, health, remote sensing, and much more. It also has a great listing of national and international GIS data catalogs. One of the best features includes a listing of US State GIS data centers.

Map Digger has an interesting listing of weather, road, nautical, historical, and aviation maps.

Online Worldwide Spatial Data