Factorise
Visualize factors through building rectangular areas on a grid. First enter all the factorizations of a number, then draw each factor set as an area on the grid. Factorize is one of the Interactivate assessment explorers.

http://www.shodor.org/interactivate/activities/Factorize/

Factorise II
Visualize factors through building rectangular areas on a grid. As you draw each factor set on the grid, the factors will be listed.

http://www.shodor.org/interactivate/activities/FactorizeTwo/

Play a "Connect-Four" style game, earning a game piece each time you answer an estimation question correctly. This game asks the players to estimate the answer to addition, multiplication, and percentage questions. Choose difficulty level, question types, and time limit. Estimator Four is one of the Interactivate assessment games.
http://www.shodor.org/interactivate/activities/EstimatorFour/

Here is a similar interactivity that doesn't have the Connect Four interface.

Estimator Quiz
http://www.shodor.org/interactivate/activities/EstimatorQuiz/

In this interactivity, you can choose 'Counting' where the students have to estimate the number of stars.

Estimator
http://www.shodor.org/interactivate/activities/Estimator/

More or Less Estimator
Is the number of objects more or less than the given estimate?
http://www.shodor.org/interactivate/activities/MoreOrLessEstimator/

Roll or choose a number in Pascal's Triangle and then click on all entries that are multiples of the number, thereby practicing multiplication tables, investigating number patterns, and investigating fractal patterns.

http://www.shodor.org/interactivate/activities/ColoringMultiples/

Arithmetic Four is a Connect Four variation where you have to answer an arithmetic question correctly before you can play a chip. You get to choose the category, the operation, the difficulty level and the time limit.

http://www.shodor.org/interactivate/activities/ArithmeticFour/?version=1....

Arithmetic Quiz is similar but without the Connect Four interface.

http://www.shodor.org/interactivate/activities/ArithmeticQuiz/

Type in a big number. Now say it. Check your answer using the website.
Now type in a really big number, say it, and check your answer.
Now type in a really, really big number, say it, and check your answer.
And so on ....

What is the biggest number you can say correctly?

http://www.mathcats.com/explore/reallybignumbers.html

Ancient Egyptians knew fractions like 1/2 or 1/4 ((one piece of a pie).
But to make fractions like 3/4, they had to add pieces of pies like 1/2 + 1/4 = 3/4.

For some reason, they never repeated the same fraction when adding; ie they never wrote: 1/4 + 1/4 + 1/4 = 3/4

This website presents a range of Egyptian maths problems for you to solve. Good luck!

http://www.mathcats.com/explore/oldegyptianfractions.html

An interactive 100 square. You can choose to start the square with either 0 or 1.
http://www.hellam.net/maths2000/100square.html

You can also change the 'width' of the 100 board using this link:
http://www.hellam.net/maths2000/nsquare.html

Here is a different 100 square. Clink on the IDEAS button to see how you might use this tool.
http://www.amblesideprimary.com/ambleweb/mentalmaths/countersquare.html

Click on two cards to turn them over. If they match, they are cleared from the board. If not, they remain on the board. How many turns will it take you to clear the board? But there is a twist - you also have to click on the 'odd number out'!

http://www.interactivestuff.org/match/maker.phtml?featured=1&id=17

Click on two cards to turn them over. If they match, they are cleared from the board. If not, they remain on the board. How many turns will it take you to clear the board?

http://www.interactivestuff.org/match/maker.phtml?featured=1&id=6

Click on two cards to turn them over. If they match, they are cleared from the board. If not, they remain on the board. How many turns will it take you to clear the board?

http://www.interactivestuff.org/match/maker.phtml?featured=1&id=5