U.S. Geological Survey (USGS)
State by state science topics, maps, recent news releases, etc.
National Geophysical Data Center
Fun, colourful site, lots of information, good for research and teaching resources.
Water Resources for the U.S.
State by state up-to-date information.
Science.gov: Earth and Ocean Sciences
A-Z links, includes narrow searches in the geology area, good for research.