A National Radiation Map that depicts environmental radiation levels across the United States is available on the home page of the Radiation Network. It is updated in real time every minute. Not a government site, this website claims to be the first where "the average citizen (or anyone in the world) can see what radiation levels are anywhere in the USA at any time."
Mineralab, LLC, who operates the web site from Prescott, Arizona, cautions that it is unable to independently verify that the radiation levels or any radiation alerts that are displayed on the Radiation Map are correct and valid. Monitoring stations are operated by grassroots volunteers. Geiger counter malfunctions or proximity of the counters to certain medical procedures or to radioactive items can cause high readings to occur.
It appears from the map that there are nine monitoring stations currently functioning. The map indicates that they are located in Washington, California, Arizona, Colorado, and Indiana with two additional stations on the East Coast.
The public is invited to join the effort. This requires a digital Geiger counter, Software available from Radiation Network, a computer with Windows operating system, and direct internet access.
Active participants receive the benefit of a map that is fully interactive: with zoom capabilities; descriptions of nuclear sites and monitoring stations; additional map layers, including counties, airports, roads, railroads, lakes and rivers; and the capability to download city streets for their home county. The software also entitles the user to participate in the network's own chat forum.
For additional information on radiation:
How Nuclear Radiation Works, from How Stuff Works
Radiation and Radioactivity, from Idaho State University Radiation Informaion Network