Some data sources (there are many more on the web):<br>
<hr>
<P>
<LI> Access to Air Pollution Data: <a href="http://www.epa.gov/air/data/index.html">  EPA AirData </a> <br>
You can query the database to obtain information. For example, I created a <a href="http://www.stat.psu.edu/~mharan/spatial/datasets/PAairqual.dat"> small PA air quality data set </a> (the latitudes and longitudes corresponding to county centroids were obtained from an online Census database). <br> 
<LI> Water Resources NSDI Node: <a href="http://water.usgs.gov/lookup/getgislist"> link </a> <br>
<!-- <LI> NASA Atmospheric datasets on Southern U.S. and Central America <a href="http://www.amstat-online.org/sections/graphics/dataexpo/2006.php">  ASA description of an old competition based on the dataset (and available data) </a>  -->
<LI> NASA Atmospheric datasets on Southern U.S. and Central America <a href="http://stat-computing.org/dataexpo/2006/">  ASA description of an old competition based on the dataset (and available data) </a> 
Some data I created from it (with columns corresponding to longitude,latitude,data):
<UL>
<LI> <a href="http://www.stat.psu.edu/~mharan/spatial/datasets/atmosphNASA/ozone1.dat"> ozone (month 1) </a> Dobson Units.
<LI> <a href="http://www.stat.psu.edu/~mharan/spatial/datasets/atmosphNASA/surftemp1.dat"> surface temperatures (month 1) </a>  Units: Kelvin (to convert to Celsius: degrees K - 273.15 = degrees C).
<LI> More details about the data are <a href="http://www.amstat-online.org/sections/graphics/dataexpo/2006data.php"> here </a>.
</UL>
<LI> The main NASA <a href="http://data.giss.nasa.gov/"> data website </a>.
<LI> Wolfcamp aquifer data set (Cressie, 1993 pg.214): <a
href="http://www.stat.psu.edu/~mharan/spatial/datasets/aquifer.dat">
aquifer data </a>. Piezometric-head data (in feet above sea level) at
85 distinct locations in the Wolfcamp aquifer of Texas and New Mexico.
</P>