National Council of Teachers of Mathematics - home page. Check out sites and resources for teachers under Teachers Corner. Check out "Do Math" problems and the Electronic Lessons, activities and products to support the principles and Standards for School mathematics.
Blue Web'n Best Sites - great sites under the math subject area search.
Cornell Theory Center - Gateway that provides links to resources in mathematics and science for educators and students grade 9-12, although teachers of all levels will find this info. helpful.
Eisenhower National Clearinghouse - is an extensive list of links to help educators find instructional materials for math and science on the Internet.
SCORE Mathematics Lessons - lessons written by teachers selected to participate in SCORE Mathematics workshops.
Calculators Online - over 5000 types of online calculators to use.
Ask Dr. Math - archive of math questions, searchable and arranged by grade level; you can also submit your own questions.
Geometry Online - interactive lessons, homework help, worksheets, puzzles, and more.
McRel : Mathematics Resources and Standards - offers a great list of links with educational standards in math included.
Totally Tessellated - a comprehensive history and explanation of these mathematical and creative products.
The Abacus : The Art of Calculating with Beads
History of Mathematics - biographies of over 1,000 mathematicians searchable by name, historical period, and country of origin.