Mad Math Minutes
addition, subtraction, multiplication, division - choose your own level
http://www.mrsbogucki.com/aemes/resource/apps/madmath/madmath.htm

Rainforest Maths
This is site has lots of interactive games for K- 6
http://www.rainforestmaths.com/

Cool Math 4 Kids
Lots of games, puzzles, various colouring pages, and resources for teachers too.
(This one has so many options it can be a bit tricky to find what you want, so be sure to bookmark any pages you like!)
http://www.coolmath4kids.com/

Maths is Fun
Math Resources
http://www.mathsisfun.com/

Learning Network - Funbrain
Baseball games, math car races etc with drill type computational questions.
http://www4.funbrain.com/

The ArithmAttack
How many computer-generated arithmetic problems can you answer in 2 minutes? Add, subtract, multiply, divide, or use a random operator.
http://www.saab.org/mathdrills/attack.html

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Teaching Time
http://www.teachingtime.co.uk/

Tangrams
Learn about them and play them
http://www.geocities.com/TimesSquare/Arcade/1335/

Tangram Webquest
http://www.besd61.k12.il.us/webquests/4th%20Grade/tangrams/

Brain Teasers of the week (gr.3-4, 5-6, 7-8)
Houghton Mifflin Education Place
Games, puzzles, board games plus a daily chess problem
http://www.eduplace.com/math/brain/index.html

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Great Maths Websites to Explore
A list of links from the "Virtual Teacher"
http://www.virtualteacher.com.au/maths.html

Ask Dr. Math
A question and answer service for math students and their teachers. A searchable archive is available by level and topic. There are also some links to online activities.
http://mathforum.org/dr.math/dr-math.html

Interactive Mathematics Miscellany and Puzzles
There is a large glossary as well as games and information about mathematics.
http://www.cut-the-knot.com/front.html

Teacher2Teacher
Aresource for teachers and parents who have questions about teaching mathematics.
http://mathforum.org/t2t/

Text only lesson plans for teachers
http://www.col-ed.org/cur/math.html

National Council of Teachers of Mathematics site (USA)
http://www.nctm.org/

Virtual Teacher
The VT site has a number of math links which are worth investigating.
http://www.virtualteacher.com.au/maths.html

The Math Forum
"The Math Forum's Problems of the Week (PoWs) are designed to provide creative, non-routine challenges for students in grades three through twelve. Problem-solving and mathematical communication are key elements of every problem."
http://mathforum.org/pow/

National Library of Virtual Manipulatives for Interactive Mathematics
Wonderful online activities to teach mathematical concepts from K - 12.
NOTE: This used to be free but now has a 7 day free trial.
http://matti.usu.edu/nlvm/nav/vlibrary.html

Tessellation Tool
Pattern Blocks
http://www.arcytech.org/java/patterns/patterns_j.shtml

Totally Tessellated
http://library.thinkquest.org/16661/
Don't skip over this opening splash screen too quickly. Take a few moments to scroll through the image gallery by clicking on the tiny Load New Images link. It has a section on M.C. Escher, the Dutch artist and father of modern-day tessellations.

What is a Tessellation?
http://mathforum.org/sum95/suzanne/whattess.html
This Math Forum page is a great introduction to tessellations for those who know some geometry.