Smithsonian for Educators
http://www.smithsonianeducation.org/

Computer-generated arithmetic problems
http://www.dep.anl.gov/aattack.htm

Brain Teasers
http://www.eduplace.com/math/brain/index.html

Basketball math
http://www.scienceacademy.com/BI/index.html

Science Fair Idea page from Elmer's Glue Inc.
http://www.elmers.com/products/sciencefair/index.asp

The Curriculum Archive of Lesson plans.  A bit commercial, but maybe some
good ideas.
http://www.buildingrainbows.com/CA/home.html

Jan Brett¹s Teacher Help-a-longs
http://www.janbrett.com/classroom_helpalongs.htm


The Math Page grades 6-12
http://www.themathpage.com/

Web Wombat Kids¹ Class Mate (prescreened links for kids)
http://www.webwombat.com.au/kidsclass/

BAM! Body and Mind
http://www.bam.gov/