The "Web" places a mind-boggling and rapidly expanding array of quality teaching resources at the fingertips of anyone with access to the Internet. Below are some recommendated sites. I've tried to restrict this list to actively managed sites with engaging materials that won't be disappearing from cyberspace anytime soon - many in fact, e.g. NASA, JPL, USGS, Eisenhower, AAAS, are truely "500 lb. gorillas" you should be able to count on as rich resources throughout your teaching career.
National Aeronautics & Space Administration Education Program
NASA Global Change Master Directory
NASA Jet Propulsion Laboratory Educational Outreach
NOAA (National Oceanic and Atmospheric Administration) Education Resources
The National Ocean Service Education & Outreach
U.S. Department of Energy Science Education
U.S. Naval Observatory (timekeepers)
Climate Change and Sea Level lesson plans from the Atmospheric Radiation Measurement (ARM) Program aimed mostly at middle school grade level.
Earth Science Lesson Plan Central
Awesome library mathematics lessons
CERES Project astronomy activities
The Gateway to Educational Materials
Earth Scientist lesson plan links
Smithsonian Office of Education lesson plans
How Volcanoes Work Vic Camps' San Diego State hosted site - selected by Scientific American as 1 or 50 top sci/tech sites in 2004!
Smithsonian Global Volcanism Program
USGS Earthquake hazards program
IRIS (Incorporated Research Institutions for Seismology) Education and Outreach
SCEC (Southern California Earthquake Center) Education and Outreach
CalTech Earthquake Information
San Diego County Water Authority Education resources
USGS Water Resource information for students and teachers
US Department of Education Federal Resources for Science Excellence
San Diego Country Office of Education
Los Angeles Country Office of Education
California State Board of Education
The New York State Education Department
National Science Education Standards (NSES)
AAAS Benchmarks for Science Literacy
California Science Content Standards Grades K-12
National Association for Research in Science Teaching (NARST)
National Science Teachers Association (NSTA)
American Association for the Advancement of Science (AAAS)
National Council of Teachers of Mathematics
National Association of Geoscience Teachers (NAGT)
ERIC Clearinghouse on Assessment and Evaluation
Field-Tested Learning Assessment Guide
Performance Assessment Links in Science (PALS)