1. The obvious – NWMI’s website.
2. Tarquin Books
They now have a series of books about paper engineering, science, optical illusions, mirror reflections, costume and history as well as do-it-yourself pop-up books and collections of colorful mobiles and gift boxes.
3. Geometry Through Art
What Children Can Learn about Art and Geometry
by Norman Shapiro (http://mathforum.org/~sarah/shapiro/shapiro.html)
This site has sample lessons for K-6 that use art to teach geometry concepts. These Web pages are meant to be a resource for educators and learners alike, providing useful, ready-made worksheets that will give students access to authentic geometric experiences. Vocabulary is organized in a continuum of grade levels, and scenarios and hands-on activities give lesson plan outlines as models for use in the classroom.
Most specifically, Geometry Through Art offers a heuristic approach for engaging learners at levels of visualization and analysis. It provides activities for developing a keener spatial sense, and offers empirical procedures that promote creativity and rigor in logic.
4. The Math Forum
The Math Forum is a leading center for mathematics and mathematics education on the Internet. The Math Forum's mission is to provide resources, materials, activities, person-to-person interactions, and educational products and services that enrich and support teaching and learning in an increasingly technological world.
5. The Geometry Junkyard
These pages contain usenet clippings, web pointers, lecture notes, research excerpts, papers, abstracts, programs, problems, and other stuff related to discrete and computational geometry. Some of it is quite serious, but I hope much of it is also entertaining. The main criteria for adding something here are that it be geometrical (obviously) and that it not fit into my other geometry page, Geometry in Action, which is more devoted to applications and less to pure math. I also have another page on non-geometrical recreational math. (Be sure to follow the recreation math link to a multitude of other links.)
6. The Geometry Center
Web site for the (now closed) Center for the Computation and Visualization of Geometric Structures
7. Native American Geometry
This award-wining site contains lessons that teach the geometry of the circle from a Native American perspective. The reference list at the end contains links to other good geometry sites.
7. Euclid Projects at the Visual Math Institute
A project of the Visual Math Institute located in Santa Cruz, California.
8. Cut-the-Knot
Learning starts from wondering, and another purpose of this site is to serve as a resource for things, simple but curious, related to Mathematics. I do not intentionally classify topics according to their simplicity. There must be an element of discovery involved to enhance a learning experience. This site is a Miscellany. A few topics are so related that they cannot be treated independently. Make your own selection that, I hope, may lead to other discoveries.
Use the table of contents to find the Geometry section. This site makes you want to retire from teaching, stay home and do mathematics day and night.
9. George Hart
This is the website of the “polyhedron man”, so named by Ivars Peterson of Science News. (And we thought Will Webber was the real polyhedron man.) This site as more you have time to know about polyhedron with links to other good sites.
10. ATM - The Association of Teachers of Mathematics (the British NCTM)
This is the source of Mat Tiles and other great materials. The specific link for materials to buy is http://www.atm.org.uk/buyonline/browsecategories.html