Note: it's probably best to visit the web site to get the links, as they may change on occasion.
National high-resolution (1 km) radar reflectivity mosaic, updated once every two minutes, can be found at
http://wdssii.nssl.noaa.gov/geotiff.
Two relevant presentations at the 2006 American Meteorological Society's annual meeting are:
Thanks,
Travis
Update 24 Aug 05: added the contrast adjustment tool suggested by "SMcQ." Re-download the link for it to take effect.
Update 28 Sep 05: added NWS warning polygons, updated once per minute. Visit
this web site to get the warning-only KMZ. Full-CONUS radar (not the tiled version) is also available there.
Update 31 Oct 05: added some GOES satellite channels and 1-hour precipitation totals. All images are full-CONUS (potentially big files).
Update 25 Jan 06: added links to background papers.
Sometime in Spring 2006: added SPC storm reports.
Disclaimer: these are provided for research only. Expect data outages.