Southwest Educational Development Laboratory
Includes content information on science and mathematics.
This site has numerous links to aid math and science teachers K-12.
The National Center for Improving Student Learning and Achievement in Math and Science
The home page for NCISLA has lots of resources for -12 math/science teachers, including many downloadable pdf articles.
The Khan Academy contains over 2,100 videos and 100 self-paced exercises and assessments, with extensive resources in mathematics and other topics covered, including biology, chemistry, physics, and earth/space science.
David Wetzel's blog is written for science and math teachers. It contains valuable resources and strategies.