United States Geological Survey (USGS)
The Incorporated Research Institutes for Seismology (IRIS)
The Center for Earthquake Research and Information (University of Memphis, TN) -- Information on central U.S. earthquakes, seismograms for area earthquakes, and maps. They also have an earthquake catalog data retrieval system for getting data on specific earthquakes or areas. Just type in some dates, a magnitude range, and a location, and it pulls up the information for you.
The University of Washington's Geophysics Department
Caltech's Seismological Laboratory
UNR Seismological Laboratory (University of Nevada at Reno) -- Earthquake information, and information on the lab and UNR. See UNRSL's Helicorder Live - their video picture of a seismograph, updated every minute. Check out the weather forecast for Reno and the UNRSL Record of the Day while you're there.
St. Louis University Earthquake Center -- New Madrid, you're on the map! Information on the New Madrid Seismic Zone of the central Mississippi River Valley, the 1811-1812 quakes in that area, and recent earthquakes (including local quakes).
MichSeis (University of Michigan Seismological Observatory) -- Information on selected earthquakes and links to other sites.
Take a look at Purdue University's seismology resources for teachers.
Institute of Geophysics, University of Stuttgart -- Here you'll find technical description of seismometers and their calibration.
More Earth Science at Purdue University -- An excellent site prepared by Prof. Lawrence W. Braile.