National Council of Teachers of Mathematics
http://www.nctm.org/

Geometry Lesson Plans
http://www.geometry.net/basic_g/geometry_lesson_plans.php

Math.com For Teachers
http://www.math.com/teachers.html

TeacherVision | Math Teacher Resources
http://www.teachervision.fen.com/math/teacher-resources/8592.html

Euclid's Elements (of course!)
http://aleph0.clarku.edu/~djoyce/java/elements/toc.html

Joseph Malkevitch Geometry for Children
http://www.york.cuny.edu/~malk/geometry-children.html

Geometry Grades 3-5
http://nlvm.usu.edu/en/nav/category_g_2_t_3.html

Geometry 3-4
http://teams.lacoe.edu/documentation/classrooms/amy/geometry/3-4/3-4.html

The Geometry Applet
http://aleph0.clarku.edu/~djoyce/java/Geometry/Geometry.html

Mathematics Journals
http://www.ams.org/mathweb/mi-journals.html

 

Parnet Guide to the Internet
http://www2.ed.gov/pubs/parents/internet/index.html

Early Childhood: Where Learning Begins - Mathematics
http://www2.ed.gov/pubs/EarlyMath/whatis.html

Helping Your Child Learn Math
http://www2.ed.gov/pubs/parents/Math/index.html

Math in the Home
http://www.kidsource.com/kidsource/content/Learnmath5.html

Frequently Asked Questions in Mathematics
http://www.cs.uwaterloo.ca/~alopez-o/math-faq/math-faq.html

 

The Geometry Center
http://www.geom.uiuc.edu/

Beginning Algebra Tutorial on Basic Geometry
http://www.wtamu.edu/academic/anns/mps/math/mathlab/beg_algebra/beg_alg_tut33_geom.htm

Geometry in Action
http://www.ics.uci.edu/~eppstein/geom.html

Topics in Geometry: Art and Geometry
http://www.math.wichita.edu/history/topics/geometry.html

Fractal Geometry
http://classes.yale.edu/fractals/

Gallery of Interactive Geometry
http://www.geom.uiuc.edu/apps/gallery.html

Geometry Problem of the Week
http://mathforum.org/pows/currentProblem.htm?group=GEOPOW

Egyptian Geometry
http://www.math.buffalo.edu/mad/Ancient-Africa/mad_ancient_egypt_geometry.html