Comprehensive mathematics resource for questions on content, links to educational resources, etc... If you click on Math Help on the top tab, and then "Ask Dr. Math," you can search previously asked questions regarding mathematics and can get explanations for a wide range of mathematical procedures, concepts, and applications.
Teaching resources for mathematics teachers, a question and answer service for teachers and students, careers of mathematicians, a problem of the month and more.
Great resouce for children's literature applicable to math lessons. You can search by grade or topic. The site also has links to some lesson plans and online activities which go with some of the books.
This site has searchable video cases of real elementary math lessons teaching standards based mathematics lessons. You can also see a complete lesson plan with handouts.