For students interested in global datasets or data about countries outside of the U.S., we have found a couple useful sites (although we have referenced others already, and certainly there are more we are not aware of).
The Global Spatial Data Infrastructure Association (http://www.gsdi.org/) is an organization of organizations around the world that promotes the development of spatial data infrastructures. Browse their site to access some of their http://www.gsdi.org/Electroninc%20Gateways.as? "gateways" to global data.
There are also sources of GIS (and useful secondary global data) on various sustainability and environmental topics by the:
World Resources Center (http://pubs.wri.org/datasets.cfm?SortBy=1)
The United Nations Environment Programme (http://www.grida.no/db/gis/prod/html/toc.htm), and
FAO's Geonetwork (http://www.fao.org/geonetwork/srv/en/main.search).
The rest of this section, presents information about some common categories of data available at global scales:
Browse through the variety of elevation products provided by the USGS at http://edc.usgs.gov/products/elevation.html.
SRTM3 (Version 2) (ftp) is one of the primary sources of gridded elevation data with global coverage. The resolution is 3 arc-second or roughly 90 meter. These data were obtained by the Shuttle Radar Topography Mission (SRTM) which used Synthetic Aperture Radar (SAR) to measure all land surfaces between +/-60 degrees of latitude. They provide a globally consistent base dataset for standard analyses that are terrain-dependent. The SRTM data suffer from data gaps in higher-relief areas, so one could patch and fill them with data from SRTM30 (30 arc-second or approximately 1km resolution), itself a blend of GTOPO30 and SRTM data. A complete assessment of the SRTM Topographic Products (pdf) has recently been released and provides important information on both absolute and relative height and locational errors in the SRTM3 data.
However, recall that you can get seamless digital elevation data for the United States and some other locations as well at http://seamless.usgs.gov/. This is very nice because you don't have to mosaic data together and the seams between datasets have been delt with.
The World Wildlife Fund's "Hydrosheds" project. http://www.worldwildlife.org/freshwater/hydrosheds.cfm. "Hydrosheds" stands for "Hydrological data and maps based on Shuttle Elevation Derivatives at multiple scales." These are data derived from the Space Shuttle topographic mission.
The USGS watershed HYDRO1K project. http://edc.usgs.gov/products/elevation/hydro1k.html This dataset contains all watersheds on a nested basis down to the resolution obtainable with 1km digital elevation grids. One could query HYDRO1K to find the location that a specific point occupies, then use the special topological encoding of the HYDRO1K basins to additionally select all upstream basins, if any. These selected basin polygons then represent the entire watershed for the site.
The Global Land Cover Facility (GLCF)] at the University of Maryland (http://glcf.umiacs.umd.edu/index.shtml) maintains multi-decadal global coverage of Landsat (http://landsat.gsfc.nasa.gov/) data. A path/row pair is needed to retrieve each image. Landsat 7 offers recent (1999 and later) coverage of all land surfaces.
Google Earth http://earth.google.com/ and some other sources on the internet provide high-resolution (approximately 1-meter) satellite imagery of some urban areas, a great resource for visual inspection.
Some of the earlier sites such as the USGS Earth Explorer provide mechanisms to find remotely sensed images as well.
The Center for Hazards and Risk Research (http://www.ldeo.columbia.edu/chrr/research/hotspots/coredata.html at Columbia University provides interesting global data sets of cyclone, drought, earthquake, flood, landslide and volcano hazard frequency and distribution, and also cyclone, drought, earthquake, flood, landslide and volcano associated mortality risks. Here is an example of global landslide data obtained from their website:
The Global Historical Climatology Network (GHCN) version 2 (http://www.ncdc.noaa.gov/oa/climate/climatedata.html#monthly provides station-based monthly precipitation totals for about 10,000 stations worldwide among other climate data.
Willmott et al. (http://climate.geog.udel.edu/~climate/html_pages/archive.html have used these and other climate information to develop a global terrestrial optimally-interpolated half-degree resolution monthly timeseries grid. They provide global data in the form of monthly air temperature, total precipitation, terrestrial water budgets and moisture indices. They also have additional climate datasets for the Arctic, Tropical and South American regions.
The World Meteorological Organization (WMO) maintains a database of selected long-term daily precipitation records at http://www.wmo.ch/web/gcos/gcoshome.html
The U.S. National Climate Data Center is another resource http://www.ncdc.noaa.gov/oa/ncdc.html
Finally, WorldClim (http://www.worldclim.org/ is a set of global climate grids with a spatial resolution of a square kilometer. The data are described in: Hijmans, R.J., S.E. Cameron, J.L. Parra, P.G. Jones and A. Jarvis, 2005. Very high resolution interpolated climate surfaces for global land areas. International Journal of Climatology 25: 1965-1978. A pdf of this paper can be found at http://www.worldclim.org/worldclim_IJC.pdf.
The World Data Center for Human Interactions in the Environment at CIESIN (http://www.gateway.ciesin.org/wdc/) provides population data and administrative boundaries. Some examples of data sets are a 5X5 minute (long/lat) gridded population of the world and World Bank data collection of 170+ countries, targeting the social effects of economic development.
Last, but not least, the dataserver diva-gis (http://www.diva-gis.org/data/DataServer.htm) provides a search mechanism for data by country around the world. Note: The last time we accessed this the search mechanism wasn't working properly.