ARC Center Study on Standards Based Curricula

http://www.comap.com/elementary/projects/arc/

AIMS Puzzle Corner – over 100 math puzzles

http://www.aimsedu.org/Puzzle/index.html

Aunty Math:Math Challenges for K-5 Learners

http://www.auntymath.com

Figure This! Math Challenges for Families

http://www.figurethis.org/index.html

The Franklin Institute Online - problem solving activities

http://www.fi.edu/math/index.html

The Freudenthal Institute in the Netherlands. This site has wonderful online games that challenge and reinforce mathematical minds.

http://www.fi.uu.nl/rekenweb/en/

TERC:

Investigations in Number, Data and Space

http://investigations.terc.edu/index.cfm

Math Brain Teasers

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

Math Forum at Drexel University

http://www.mathforum.com/

Mathematically Sane - a resource for information on standards- based math curricula.

http://www.mathematicallysane.com/home.asp

NCTM Illuminations

http://illuminations.nctm.org

PBS Kids Cyberchase

http://pbskids.org/cyberchase/

The SET Game

http://www.setgame.com

Virtual Manipulatives

http://nlvm.usu.edu/en/nav